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warm cropping of steel billets
Journal of Mechanical Working Technology, 2 (1978) 217--239
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© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
FACTORS AFFECTING STRESS CRACKING IN COLD/WARM CROPPING OF STEEL BILLETS*
B.B. BASILY and M.K. DAS
Mechanical Engineering Department, Birmingham University, Birmingham (England) (Received May 17, 1978)
Industrial Summary In the forging industry, the traditional method of billet production by sawing is being replaced increasingly by shearing or cropping, as the latter offers substantial economic advantages. A considerable range of steels can be cropped satisfactorily in the cold condition for use by the forging industry. However, a number of steels of specific composition, which are used currently for hot-forged automobile components, cannot be cropped satisfactorily by the conventional cold-cropping method. This is because the billets cropped from these materials tend to exhibit stress cracks on the cropped faces, making them unsuitable for forging. This type of billet defect appears to be the most difficult to prevent, particularly as the cracks may not be visible immediately after shearing and can remain undetected for several hours/days. These delayed cracks and other billet defects are influenced by several factors, which fall into two main groups. The first of these is metallurgical and primarily relates to the chemical compositions, existing impurities and the physical properties of the bar stocks in as-received condition. The second group consists of mechanical effects such as billet geometry, type of cropping machine and the characteristics of the tooling employed. Both groups of factors are interrelated and the proper selection of the latter is, often, guided by the former. One of the ways of producing crack-free billets is to preheat the bar stock before the cropping operation, which acts primarily to provide an increase of the material ductility and/or improved impact properties. However, it appears that current preheating practice varies widely within the forging industry without any apparent scientific basis. The present paper examines the relevant factors in order to establish their respective influence on stress cracking. Their identification is necessary if guide lines are to be established for either the efficient use of the thermal energy consumed in preheating, or possibly its total elimination through better cropping techniques and improvements in the as-received bar stocks.
Introduction I n the past, m o s t forgers used to p r o d u c e crops or discrete lengths of steel b a r s f o r u s e i n f o r g i n g d i e - s e t s b y s a w i n g . W i t h t h e e x c e p t i o n o f v e r y large *Paper presented at the International Conference on Warm Working, Sunderland, U.K., September 11--12, 1978.
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cross-sections, sawing has now being replaced b y shearing or cropping. The reasons for this change-over to a chipless forming method are twofold, and result in substantial economy. Firstly, in cropping, the crops or the billets are produced b y a process of controlled f r ~ t u r e and there is therefore no material wastage. Secondly, cropping is a highly productive method ideally suited for the large quantities required b y the forging industry. It is well k n o w n that the quality of the cropped billets is inferior to that of those produced b y sawing. The former exhibits various kinds of distortions of a geGmetric nature and some metallurgical defects such as work-hardening. The demand for metallurgical billet quality for hot forging is minimal as long as the geometric distortions are within certain limits -- that is, if feeding into the die-set and the metal flow or the loading patterns are not seriously affected. Since the billets for hot-forging application are heated well b e y o n d their recrystallization temperature, some of the metallurgical defects do not pose a serious problem. However, one particular t y p e of metallurgical defect makes a billet unsuitable for further processing even in hot forging. This takes the form of surface defects on the cropped faces such as cracks, splintering and/or the formation of cheeks, as shown in Fig. l(a), (b) and (c), respectively. These defects are b y no means universal and are usually found in high-strength steels with large carbon and alloying contents in which impurities such as hydrogen or banded inclusions are present. One particular feature of these stress cracks, which makes them particularly unwelcome, is the fact that they remain undetected
a . DIAGONAL CRACKING
b.
SUB-SURFACE CRACKING AND SPLINTERING
C. FORMATION OF CHEEKS
Fig. 1. Various stress cracking defects in cropped billets.
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for hours or even days after shearing, as there is an incubation period before they b e c o m e visible. For this reason, terminology like "delayed crop cracking" or "delayed stress cracking" are often used to distinguish them from immediately apparent cracks usually associated with improper tool settings. The other interesting feature of this phenomenon is that larger cross-sections (usually greater than 65 mm side square bars} are more prone to stress cracking even though nominally the same composition is used and the cracking is invariably present on only one of the newly created cropped surfaces. Steels of this kind, therefore, are not readily shearable in the cold state (ambient temperature). The current practice in industry involves the use of a furnace to preheat the bar stock in order to crop it in a warm or hot state which, under certain conditions, eliminates the problem of stress cracking on the cropped faces. The relative increase in the cost of billet production due to the need to preheat the bar stock was not very significant [1] in relation to over-all costs prior to the recent sharp rise in energy costs. However, this rise has produced a profound effect on industry and indiscriminate use of energy is being curtailed, not only through effective insulation but also through rational deployment. It would appear, therefore, that if, through better understanding of the process, hot cropping is made more efficient in terms of thermal energy consumed, a considerable saving will ensue. Apart from the running costs, hot cropping also needs extensive capital equipment and a large amount of floor space. Thus, its total elimination, should it be possible through better cropping techniques and improved bar stock in the as-received condition, is highly desirable. The present paper reports some of the preliminary work carried out to gain an understanding of the relevant parameters involved in stress cracking of cropped billet faces. It will be useful to start the discussion by considering the effects of various factors on the billet quality, starting right at the level of material composition. Factors affecting cropped billet quality The quality of cropped billets is assessed in two ways: the geometrical distortions as measured in either absolute or relative terms and the metallurgical defects as expressed in terms of work-hardening or residual stresses. In extreme cases, the metallurgical faults appear in the form of visual cracks on the cropped faces. The metallurgical defects are related to the geometric faults in the sense that large distortions usually accompany severe metallurgical faults, depending on the stress--strain characteristics of the stock material. Metallurgical defects are further affected b y the fracture process involved in cropping, as determined b y the operating conditions. Therefore, in general the quality of the cropped billet is affected b y t w o groups of parameters, namely, metallurgical and mechanical, as shown in Table 1. The former includes chemical composition, strength (hardness), micro-structure, fracture characteristics, impurities and the method of manu-
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TABLE 1 Factors affecting the quality of cropped billets Group
Parameters
Main influence
Composition
Mainly C% and alloy content in determining strength and fracture characteristics.
Strength (hardness)
Less billet distortions in high-strength materials but affects cold shearability at very high tensile strengths.
Microstructure and segregation
Grain size and inclusions affecting the quality of the cropped faces (usually worse with larger-size bars).
Fracture charactenst~s
Brittle/ductile transition etc. resulting in stress cracking at ambient temperatures.
Impurities
Hydrogen embrittlement of cropped faces.
Method of manufacture
Rate of cooling affecting above physical properties and impurities.
Cross-sectionai area
High loads, leading to distortions and residual stresses etc.
Bar face orientation
Less geometric distortions with increased sectional rigidity (e.g. diagonal cropping of square billets).
Specific billet length or the aspect ratio (L/D)
Billet quality worsens as the aspect ratio drops.
Capacity
Near stalling situation leading to billet defects in low-capacity machine.
Tool stroke
Double-shear or "follow-through" defects with unnecessary long stroke.
Speed
Less distortions and cleaner crops at high speeds.
Rigidity
Affects tool clearances etc.
Blade design
Type of blades (e.g. " o p e n " type blade produces inferior crops as compared to "closed" type blades).
Cutting edge
Dull edges cause delay in fracture initiation resulting in high distortion.
Bar-holding (clamping)
Reduces distortions and variation in billet weight.
Off-cut support
Reduces bending and improves billet quality.
Aligmnent and clearances
Optimised conditions for a material and bar size yield better billets.
Metallurgical Material
Mechanical Billet geometry
Machine
Tooling
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facture involved in the primary process of rolling. The mechanical group of parameters can, further, be divided into three sub-groups: billet geometry, machine, and tooling characteristics. The right-hand side of Table 1 indicates the main influence of these parameters as reflected in the quality of the cropped billets. These are discussed below in more detail with particular reference to their role in stress cracking of the cropped faces. Metallurgical parameters
(a) Chemical composition: The chemical composition of the bar stock has great influence on the quality of the cropped billets, and is a critical factor in deciding the cold cropability of the material using conventional cropping methods. A survey of the current practice referred to b y Marston [1] shows that most carbon steels can be cropped cold, provided that the carbon content does n o t exceed 0.5%. On the basis of these findings, Marston was able to divide the majority of En series and other steels into two groups: (i) material which may be cold cropped and (ii) material which requires preheating, as shown in Table 2. The suggested temperature range for obtaining satisfactory crops in most steels on the right-hand side in Table 2 is between 300 and 400°C. The importance of the carbon content in deciding whether a steel should be cold or hot cropped can be seen from Figs. 2, 3 and 4. In all cases, square 6
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TABLE 2
Division of materials f o r c o l d a n d h o t c r o p p i n g ( a f t e r Ref. 1) Material which may b e c r o p p e d cold
Material which requires preheating
En 1 En 2 En 3 En 5 En 8 E n 14 E n 16 u p t o 2.25 in 2 E n 18 u p t o 2.5 in 2 E n 24 u p t o 1.75 in 2 E n 32 E n 35 E n 36 u p t o 2.75 in 2 E n 39 u p t o 2 in 2 En 43 E n 1 1 0 u p t o 2.5 in 2 En 201 E n 351 En 352 E n 3 5 3 u p t o 2.5 in ~ En 354 En 361 En 362 S.A.E. 1 0 4 2 S.A.E. 1 0 4 6 S.A.E. 8 6 1 5 S.A.E. 8 6 2 0 S.A.E. 8 6 2 2 S.A.E. 4 6 2 0
En 9 E n 17 E n 18 a b o v e 2.5 in: E n 19 E n 24 a b o v e 1.75 in s En 26 E n 39 a b o v e 2 in s E n 40 En 100 E n 100 a b o v e 2.5 in 2 E n 353 a b o v e 2.5 in 2 S.A.E. 8 6 4 0 S.A.E. 4 1 4 0
points indicate the hot-cropped steels whereas circles represent the cold-cropped steels. It is interesting to note that the hot-cropped steels contain more than 0.3% C and lie on the right-hand side of the line AB (Figs. 3 and 4). An examination of the effect of the content of the alloying elements on the need to preheat did n o t show any specific pattern. The only exception was nickel content, as can be seen from the graph plotted in Fig. 2. Some of the steels with a high nickel content have a carbon content of less than 0.4%, b u t need preheating to avoid stress cracking*. It m a y be added that some forgers have reported stress cracking with steels having a high manganese content (over 1% Mn). * T h e o n l y e x c e p t i o n in Fig. 2 is E n 3 5 3 w h i c h n e e d s p r e h e a t i n g d e s p i t e its l o w c a r b o n a n d n i c k e l content. This is due to the presence o f banded segregation in the m i c r o - s t r u c t u r e o f t h e m a t e r i a l , as e x p l a i n e d in Ref. 3.
223
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(b) Material strength (hardness): In general, neither very soft nor very hard materials respond favourably to cropping. The former produces large billet distortions while the latter tends to increase the metallurgical defects. Following the work reported by Marston [ 1 ], who drew the line of hot cropping at a UTS of about 770 N/mm 2 (50 tonf/in2), Thomas [2] conducted a comprehensive survey to assist drop-forgers in deciding which material will require preheating. This was attempted by plotting data covering a range of UTS as a function of carbon content, in order to divide steels on the basis of UTS and carbon content. However, as shown by the insert to Fig. 3, this graph showed considerable scatter and no clear indication of UTS effect on the need to preheat could be obtained. When more data on En series and other steels were added, the scatter appeared even more pronounced with respect to the assumed boundaries, as can be seen from Fig. 3. However, a dividing line (AB) can easily be drawn on the basis of carbon content alone. Therefore, the indications are that UTS is not a criterion on which division can be made for cold and hot cropping of steels.
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( c ) Micro-structure and segregation: These parameters essentially provide the history of rolling and subsequent cooling as employed in the primary process of bar stock manufacture. By microscopic examination of the material, certain conclusions can be drawn with regard to the cold cropability of the material. In certain cropped specimens of En 353 exhibiting stress cracking, the reason for the defect was found to be in the micro-structure [3], where segregation was more pronounced towards the billet axis. A closer examination of a section through the axis of the specimen showed that the stress crack had progressively jumped from one bainite band to another. Axial segregation of another type, i.e. a non-metallic inclusion stringer, has also been found to be responsible for triggering stress cracking in certain cases [3]. As discussed later in connection with size effects, the grain-structure and inclusions can be significantly different in sections of different sizes, although nominally the same chemical composition is maintained (see Figs. 7 and 8). It must be added that microscopic examination has limitations, as discussed by Wannel [4]. The primary disadvantage is the total volume of material
225
actually surveyed, which is still comparatively small even with the aid of a modern quantitative microscope covering a large number of fields. Further, it takes a considerable time to prepare specimens and the results are prone to subjective judgements. However, this kind of study can be used for qualitative evaluation. (d) Fracture characteristics: The problem of stress cracking is more pronounced during winter months and in practice (Continental and American forgers) the preheating temperature achieved through steam or boiling water can be as low as 80--90°C. Steel suppliers often warn their customers to ensure that during winter the stock is stored indoors for a suitable length of time prior to cropping [4]. It is unlikely that such temperature changes will bring a b o u t significant tensile property variation. However, such a temperature change can cause a considerable alteration in impact properties, as pointed out by Thomas [2] in connection with a study of the problems associated with stress cracking. Thomas' reasoning was that if low preheating temperatures are effective in improving shearing characteristics, there should be some correlation between shearing behaviour and impact strength. To test this hypothesis, he plotted the Izod impact values and the carbon contents for a few steels. As shown in the insert to Fig. 4, this led to the conclusion that an Izod value of 40 ft-lbf (54 Joules) is required for safe cold cropping. However, when a lot more data are added, as shown in Fig. 4, there is no correlation between the impact strength and hot cropping of steels, as a number of steels with an Izod impact strength lower than 40 ft =lbf can be cold cropped and vice versa. In fact, the dominant parameter seems to be the carbon content (greater than 0.3%) and as is clearly seen in Fig. 4, the steels on the right-hand side of the line AB require preheating. The negative conclusion drawn from the plotting of Izod impact strength of steel, however, does not disprove the hypothesis that impact properties play an important role in stress cracking. As will be discussed later, in connection with the effects of preheating, the brittle/ductile transition curves of steels indicate that if preheating temperature is raised well b e y o n d the transition temperature, stress cracking is eliminated [5]. (e) Impurities: The most serious impurity, with reference to stress cracking of cropped billets, is the presence of hydrogen. High-sulphur steels are also prone to this defect [2]. The role played b y hydrogen content and hardness has been investigated b y the British Steel Corporation [4] through monitoring the hydrogen present in original casts and subsequent cold cropped billet quality as reported b y users. Fig. 5 shows the relationship between original hydrogen content in the steel casts and the as-rolled hardness, in identifying the crack susceptibility. The results presented in Fig. 5 confirm that hydrogen embrittlement resulting in the stress cracking of billet ends is more likely to occur with high hardness (therefore, high strength). From this study, it was concluded that appropriate cooling procedure should be employed after rolling in order to reduce the amount of hydrogen, for improving the cold shearability of steels. This aspect of the problem is further discussed in the
226
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next section while discussing the role of the method of manufacture of the bar stock. The effect of the hardness {strength) in increasing the susceptibility of hydrogen embrittlement is also supported by the analogous finding relating to the failure of electro-plated fasteners, especially of high tensile bolts (tensile strength greater than 1000 N/mm ~ or 65 tonf/in ~) [6]. The risk of failure by delayed cracking as a result of hydrogen embrittlement was found to increase with material hardness (or strength) as can be seen from the results presented in Fig. 6. Admittedly, in this case the hydrogen is artificially induced during the electro-plating process itself. (f) Method of manufacture: This relates to the influence of the controlling parameters during the process of rolling and the thermal history immediately thereafter. In the present context, controlled cooling is required to allow the hydrogen to diffuse [4], especially in large sections of steels with high alloy
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227
content. It is k n o w n that hydrogen removal is most effective at a temperature just below the transformation temperature (around 600--650°C) in ferritic steels. This can be achieved b y employing a slow rate of cooling through the use of a pit immediately on completion of rolling. For this reason, it is a common practice amongst forgers to order steels for cropping in the pit-cooled condition. However, it should be noted that pit-cooled material need not necessarily be in a softer condition [2], since the hardness depends on the temperature at which the steel left the rolling mill table and it is not uncommon for steels to have undergone some degree of hardening. Further, pitcooled materials are, as expected, slightly more expensive than bed-cooled steels, thc~ugh they are nowhere near the cost of annealed stock. As far as the presence of defects (such as segregation, piping, surface faults etc.) is concerned, the rolling practice is regarded as primarily responsible. The quality of the original cast, the reductions given in each rolling pass, and the number of passes given between turning the stock, all influence the final product and the faults are significantly more pronounced for larger crosssections due to the relatively smaller number of passes. Mechanical parameters
(i) Billet geometry: As listed in Table 1, the important parameters determining the billet quality in this sub-group are the cross-sectional area, the bar face orientation with respect to the cropping direction, and the specific billet length or aspect ratio (L/D). The effect of the cross-sectional area or billet size is experienced in two ways. Firstly, although the gross cropping load increases linearly with area, due to the nature of the pressure distribution on the bar-tool contact lengths, the generated stress field near the cropping plane is more diffused for larger cross-sections. This results in an inferior fracture surface with relatively higher out-of-flatness (concavity) [7]. Secondly, as discussed earlier, the quality of the bar stock could be relatively worse for larger cross-sections in the asreceived condition, even though the composition is nominally the same. A typical example of this was found in specimens of En 43 C for square bars of different cross-sections when t h e y were examined microscopically. The larger size bar -- of 4 in (101.6 mm) side -- was cut into a grid of sixteen squares in order to observe whether the micro-structure and the inclusions were uniform over the whole cross-section. Incidentally, the piece of the bar under observation was taken from a sample showing stress cracks in the cropped billet ends. Fig. 7 (a) and (b) show the micro-structure and the inclusions of the sixteen squares grid, respectively. The specimens were polished and surface treated with Nital for the former. It is interesting to note that, as shown in Fig. 7 (a), the grain size was coarser in approaching the centre of the bar, indicating a lack of homogeneity. As far as the inclusions are concerned, as indicated in Fig. 7 (b), the particle sizes were considerably larger and more densely populated around the bar axis. Both of these factors are undesirable as they contribute to crack susceptibility.
228
Left-half of specimen Fig. 7. M i c r o s c o p i c e x a m i n a t i o n o f E n C steel o f 2 in x 2 in ( 5 0 . 8 m m × section.
50.8 r a m ) cross-
229
Right-half of specimen
230
Left-half of specimen Fig. 7 (continued).
231
Right-half of specimen
232
Similar microscopic examination of the smaller size bar -- of 1.5 in {38.1 mm) side -- of En 43 C showed t h a t the section was homogeneous with regard to both micro-structure and inclusions. Further, as shown in Fig. 8 (a) and (b), the grain size was very fine and the inclusions were barely visible with a very sparse distribution.
Fig. 8. (a) m i c r o - s t r u c t u r e a n d ( b ) i n c l u s i o n s o f E n 4 3 C o f 1.5 in x 1.5 in ( 3 8 . 1 m m x 38.1 m m ) cross-section.
233
Although the results discussed above are based on experience with En 43 C, the trend is likely to be true for most steels which presently require preheating. This is further supported by the fact that there is a recognised "size effect", in so far as most of the preheating requirements are only for larger crosssections (usually above 2.5 in side [1,2] and in some cases larger than 4 in side square bars [8] ). As far as the bar face orientation in relation to the cropping direction is concerned, this is only relevant for square bar, which is the t y p e of section most frequently used b y forgers. The practice of using V-blades for cropping in the diagonal direction is widespread. A recent study [9] has shown that the billets produced b y V-blades are superior to those produced b y flat blades, in that far less distortion is produced. It is assumed that this results from an increase in the load carrying capacity of the section as well as from the gradual deceleration of the tool (gradual loading) when the section is diagonally disposed. However, the depression on the cropped face or the outof-flatness tends to be slightly higher in diagonal cropping than in flat cropping
[lO]. The effect of the specific billet length or the aspect ratio (L/D) on the billet quality is well known. As long as the aspect ratio does not fall below a certain value (LID = 1.5 approx, in conventional single cropping), its influence is not serious. However, with decreased aspect ratio i.e. when a relatively shorter length of billet is cropped, the billet quality deteriorates rapidly. Apart from geometrical distortions, the degree of out-of-flatness or concavity of the cropped face becomes increasingly more marked. Thus, the crack susceptibility of the end faces should increase if relatively shorter billets are cropped. With the comparatively higher prices for lower size steel bars, there is a tendency to use larger sizes with a corresponding reduction in billet length. However, this practice could accelerate the cracking problem, unless of course an improved cropping technique -- such as double cropping -- is employed, which allows the use of an aspect ratio as low as 0.4 without any marked deterioration in billet quality [7 ]. (ii) Machine parameters: Mechanical presses are customarily used b y forgers for the production of cropped billets. The capacity of a cropping machine is primarily defined b y the maximum number of useful strokes per minute it can deliver at the maximum section size of the bar stock. Obviously, the maximum size of the bar stock will depend on the strength of the material. Thomas [2] has produced some useful nomographs showing the relationship b e t w e e n strength and maximum size shearable for different materials. In the context of stress cracking in billet ends, it is important to avoid stalling, or any sharp fall in the cropping speed, as this can lead to the tearing mode of failure of the unsheared central portion of the bar cross-section, which is the area most sensitive to stress cracking. Since the problem of stress cracking is almost non-existent for smaller size bars, there is a growing suspicion that uniform speed of cropping is partly responsible for good crops. It is worth mentioning that the tool stroke does not adversely affect the billet quality unless the travel is too long, in which case it produces a char-
234
acteristic "double shear" or "follow-through" defect. As far as the influence of the cropping speed is concerned, an increase in speed improves the billet quality in all respects. However, mechanical croppers have a limited capacity for increase of speed by any appreciable degree. Recent work [11] on high speed cropping (above 5 m/s) has shown that it is possible to obtain billets of excellent geometric shape, maintaining close weight tolerances and minimal metallurgical defects. High speed cropping, it is suggested, is also likely to eliminate stress cracking. However, the hardware presently available is only suitable for round bars of limited cros~sections (within 3.5 in -- 88.9 mm -- diameter). A cropping machine is supposed to be rigid enough to prevent undue deflections, as these can cause dynamic alteration of the set parameters such as blade clearance. With very large size bars involving high cropping loads, machine rigidity can be a problem. There are as yet no data available in the literature on the effect of machine flexibility on billet quality and so its influence on stress cracking cannot be assessed. (iii) Tooling parameters: The most important among these parameters is blade design and the provisions for bar and billet support during cropping. Optimum performance is affected by the condition of the cutting edge, its alignment, and its clearance setting, but these are essentially controllable. Guidance on cropping blades and machine settings is usually supplied by the machine manufacturers and is also available in the literature. The design of the cropping blade is dependent on the type of machine and the way the moving blade holder is arranged. In self-contained toolsets, the moving blade holder is not attached to the moving assembly of the press and in such cases a "closed'. type blade is compatible with feeding requirements, producing superior quality billets [11]. However, the cropping of square bars in mechanical presses requires a reciprocating moving blade carrier permanently attached to the moving assembly. In such cases, the moving blade design is usually in the shape of an inverted V for cropping the bar in the diagonal direction. This arrangement provides a half enclosure for the off-cut but at the fixed blade side there can be bar holding with efficient resistance to bending. Some shearing machines are supplied with an upholder to provide additional billet support, but forgers tend to ignore this as it limits the available energy, thereby reducing the size of the section that can be cropped by about 20% [2]. However, an upholder should be used as it improves the billet quality and thus reduces susceptibility to stress cracking. No comparable data are available showing the effectiveness of upholders in relation to stress cracking in cropped billet ends. Effect of preheating As discussed earlier, the purpose of preheating the bar stock is to reduce or eliminate the chances of stress cracking in cropped billet ends. The implication is that preheating must influence the properties of bar stock in a manner
235
conducive to the purpose. In a parallel, though not analogous, process (warm forging), the bar stock is heated to within the temperature range 300--800°C with the aim of creating lower loads and energies than are usual with cold forging. A second aim is to eliminate the need for special billet preparation such as phosphating [12]. In the case of warm/hot cropping, however, the reduction of load and energy is n o t of primary importance although it certainly contributes to the maintenance of a relatively uniform speed. An early -- 1902 -- reference on hot cropping can be found in the work of Codron [13] who observed an increase in material ductility leading to an improvement in billet quality when the bar stock was heated to 1000°C. In particular, the penetration depth was increased with a corresponding reduction of the fractured zone. Cropping tests [3] conducted within the temperature range 80--300°C on 2.5 in square bars of En 353, showed that these preheat temperatures produce less distortions in shear planes. As shown in Fig. 9, when the preheat temperature was progressively increased, the degree of outof-flatness of the cropped faces improved: that is, the depression suffered b y the concave face became lesser in magnitude. The above finding is significant for the stress cracking aspect of billet quality. Cropping generates two surfaces, one on the bar end side which is largely concave and the other on the off-cut side which is (matching) convex. It is rare to find both these surfaces exhibiting stress cracks and almost invariably, it is the concave face which is prone to stress cracking. Fig. 10 shows the opposite cropped surfaces with a typical diagonal crack running through the concave face [ 3]. Thus, the effectiveness of preheating partly
MATERIAL
En 353,
SIZE: 2.5 in sq.
5.0mm
PREHEAT TEMPERATURE
8o~c ~ 3.0mm I 70°C
f
3oo°c
Fig. 9. Effect of preheat temperature on the cropped face concavity (after Ref. 3).
236
L~
cm I
Fig. 10. Opposite cropped surfaces with a typical diagonal crack running on the concave face (after Ref. 3). lies in i m p r o v i n g t h e q u a l i t y o f t h e c o n c a v e face t h e r e b y i n h i b i t i n g t h e c r a c k growth. A c c o r d i n g t o C a r t w r i g h t [ 5 ] , t h e r e a s o n w h y t h e c o n c a v e s h e a r f a c e is t h e o n e t h a t cracks is t h e p r e s e n c e o f relatively higher residual tensile stresses a t
237
the centre of the face. He found support in Scheuermann's [10] results on residual stresses showing 20% more residual stress in the concave end as compared to the corresponding convex face. Further, Scheuermann concluded t h a t residual stresses in the vicinity of the cropped face decrease with the rise in preheat temperature. Some of the residual stress values experienced a drop of about 50% between room temperature and 450°C. Even for a low temperature case when the bar stock was heated in a hot water bath, a small reduction of the residual stresses was found. However, Scheuermann's results should be treated with caution as the method employed for the measurement (strain gauge rosettes) is not entirely satisfactorily. Also, for accurate determination of the residual stresses, the cropped faces should be as flat as possible, a requirement which c a n n o t be met in practice. The Drop Forging Research Association's research programme [5] on stress cracking included the compilation of data on the impact properties and relative brittleness of the bar stock over a given temperature range. The intention was that the data collected might be used to predict minimum preheat requirements and also to develop simple room temperature test sequences for use by drop forgers to identify risk of stress cracking. Although it remains to be seen whether such definitive answers to the problem can be found, certain interesting results have emerged, such as the relationship o f impact strength and brittleness to the temperature of the stock and the occurrence of stress cracks, as shown in Fig. l l ( a ) and (b) for En 353 and En 43 B steels. Both
^
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Fig. 11. Effects o f temperature on impact strength and ductility o f steels p r o n e to stress cracking: (a) En 353, (b) En 43 B (after Ref. 5).
280
238
these steels are prone to stress cracking in the cold crop condition. At temperatures b e y o n d the brittle/ductile transition range, the specimens tested were free from stress cracking, the implication being that the preheating temperature must be well above the transition temperature. One more conclusion has been derived from the D F R A work: the fracture appearance of bar stock provides a better indication of crack risk than energy absorption, the shape and slope of the curves being very uniform. Lastly, there have been some reports of Russian work [14] on h o t cropping of high strength steels, in connection with very short billet cropping under axial compressive stresses. The cropped faces, which showed micro-cracks for the cold condition, were free from cracks when the temperature of the bar stock was raised to 400--700°C. This was attributed to an increase in material ductility, and a drop in b o t h the cropping load and the required axial compressive stresses was also observed with the rise in temperature. Conclusions
From the present study, it appears that as far as the stress cracking of billet ends is concerned, metallurgical parameters are as important as mechanical parameters. Metallurgical parameters can be controlled through choice of rolling technique, subsequent thermal treatment, and a reduction of hydrogen c o n t e n t either in the original cast and/or b y subsequent diffusion. The control of mechanical parameters involves choice in alignment, cropping speed, billet support etc., all of which determine the degree of distortion. D o u b l e cropping is an example of controlling certain mechanical parameters, such as bar and billet support, yielding better crops. Fig. 12 shows typical improvements in billet quality from double cropping when compared with the traditional m e t h o d of single cropping. A crucial factor is concavity depth which, if reduced, should reduce the risk of stress cracking. As shown in Fig. 12 this factor is reduced by half for the case of double cropping. Further, the strainhardening is considerably reduced as evidenced from the plotting of iso-hardness lines in Fig. 12, representing the relative rise over the base hardness of the material. Combination of these t w o factors ought to minimise the chance of stress cracking. Improvement can also be gained through the use of high cropping speeds. Cropping machines should n o t be used at the t o p end of their capacities and preferably an over-rated machine should be chosen, as the changes in cropping speed will be smaller. On the question of more efficient use of thermal energy, there is, as yet, no rational basis other than the trial-and-error approach, though it is fairly well established that high-risk steels can be identified on the basis of carbon content. High-strength steels are more prone to stress cracking simply because hydogen embrittlement is more pronounced in them, even at minute levels.
239
SINGLE CROPPING
DOUBLE CROPPING { M [d d',~. Crop;
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Acknowledgements The work reported here forms a part of a comprehensive programme of research on "Metal Forming and Associated Areas" supported b y a Consolidated Research Grant from the Science Research Council (U.K.), and under the general guidance of Professor S.A. Tobias, to w h o m the authors are thankful.
References 1 G.V. Marston, Met. Treat. Drop Forg., November (1963) 437. 2 A. Thomas, Metall. Met. Form., 41 (1974) 198. 3 Anon., D F R A Report No. RC 75/145, Drop Forging Research Association, Sheffield, 1975. 4 P.H. Wannel, Metall. Met. Form., 43 (1976) 207. 5 B. Cartwright, Delayed crop cracking, to be published. 6 D.G. James, Plating News, 3M Publication, April (1978) 6. 7 M.H.M. Ahmed, A study of the multiple cropping process, Ph.D. Thesis, University of Birmingham, 1978. 8 W.C. Tucker, Steel Process., November (1954) 695. 9 M.H.M. Ahmed and M.K. Das, An experimental study of the double cropping process, to be presented at the 19th MTDR Conf., Manchester, September 1978. 10 H. Scheuermann, Ind. Anz., 97 (1975) 1538. 11 M.K. Das and S.A. Tobias, Metall. Met. Form., 43 (1976) 47. 12 S.E. Rogers, Metall. Met. Fo~m., 43 (1976) 36. 13 C. Codron, Experiments on the work of the machines used foo' metals (in French), Vols. I and II, Dunod et Pinat, Paris, 1902 and 1906. 14 G. Tcemann, Povyshenie Tochnosti i automatizatsiya shtampovki i kovki, Book No. 9, edited by V.T. Mesherin, Moscow, 1971.
217
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
FACTORS AFFECTING STRESS CRACKING IN COLD/WARM CROPPING OF STEEL BILLETS*
B.B. BASILY and M.K. DAS
Mechanical Engineering Department, Birmingham University, Birmingham (England) (Received May 17, 1978)
Industrial Summary In the forging industry, the traditional method of billet production by sawing is being replaced increasingly by shearing or cropping, as the latter offers substantial economic advantages. A considerable range of steels can be cropped satisfactorily in the cold condition for use by the forging industry. However, a number of steels of specific composition, which are used currently for hot-forged automobile components, cannot be cropped satisfactorily by the conventional cold-cropping method. This is because the billets cropped from these materials tend to exhibit stress cracks on the cropped faces, making them unsuitable for forging. This type of billet defect appears to be the most difficult to prevent, particularly as the cracks may not be visible immediately after shearing and can remain undetected for several hours/days. These delayed cracks and other billet defects are influenced by several factors, which fall into two main groups. The first of these is metallurgical and primarily relates to the chemical compositions, existing impurities and the physical properties of the bar stocks in as-received condition. The second group consists of mechanical effects such as billet geometry, type of cropping machine and the characteristics of the tooling employed. Both groups of factors are interrelated and the proper selection of the latter is, often, guided by the former. One of the ways of producing crack-free billets is to preheat the bar stock before the cropping operation, which acts primarily to provide an increase of the material ductility and/or improved impact properties. However, it appears that current preheating practice varies widely within the forging industry without any apparent scientific basis. The present paper examines the relevant factors in order to establish their respective influence on stress cracking. Their identification is necessary if guide lines are to be established for either the efficient use of the thermal energy consumed in preheating, or possibly its total elimination through better cropping techniques and improvements in the as-received bar stocks.
Introduction I n the past, m o s t forgers used to p r o d u c e crops or discrete lengths of steel b a r s f o r u s e i n f o r g i n g d i e - s e t s b y s a w i n g . W i t h t h e e x c e p t i o n o f v e r y large *Paper presented at the International Conference on Warm Working, Sunderland, U.K., September 11--12, 1978.
218
cross-sections, sawing has now being replaced b y shearing or cropping. The reasons for this change-over to a chipless forming method are twofold, and result in substantial economy. Firstly, in cropping, the crops or the billets are produced b y a process of controlled f r ~ t u r e and there is therefore no material wastage. Secondly, cropping is a highly productive method ideally suited for the large quantities required b y the forging industry. It is well k n o w n that the quality of the cropped billets is inferior to that of those produced b y sawing. The former exhibits various kinds of distortions of a geGmetric nature and some metallurgical defects such as work-hardening. The demand for metallurgical billet quality for hot forging is minimal as long as the geometric distortions are within certain limits -- that is, if feeding into the die-set and the metal flow or the loading patterns are not seriously affected. Since the billets for hot-forging application are heated well b e y o n d their recrystallization temperature, some of the metallurgical defects do not pose a serious problem. However, one particular t y p e of metallurgical defect makes a billet unsuitable for further processing even in hot forging. This takes the form of surface defects on the cropped faces such as cracks, splintering and/or the formation of cheeks, as shown in Fig. l(a), (b) and (c), respectively. These defects are b y no means universal and are usually found in high-strength steels with large carbon and alloying contents in which impurities such as hydrogen or banded inclusions are present. One particular feature of these stress cracks, which makes them particularly unwelcome, is the fact that they remain undetected
a . DIAGONAL CRACKING
b.
SUB-SURFACE CRACKING AND SPLINTERING
C. FORMATION OF CHEEKS
Fig. 1. Various stress cracking defects in cropped billets.
219
for hours or even days after shearing, as there is an incubation period before they b e c o m e visible. For this reason, terminology like "delayed crop cracking" or "delayed stress cracking" are often used to distinguish them from immediately apparent cracks usually associated with improper tool settings. The other interesting feature of this phenomenon is that larger cross-sections (usually greater than 65 mm side square bars} are more prone to stress cracking even though nominally the same composition is used and the cracking is invariably present on only one of the newly created cropped surfaces. Steels of this kind, therefore, are not readily shearable in the cold state (ambient temperature). The current practice in industry involves the use of a furnace to preheat the bar stock in order to crop it in a warm or hot state which, under certain conditions, eliminates the problem of stress cracking on the cropped faces. The relative increase in the cost of billet production due to the need to preheat the bar stock was not very significant [1] in relation to over-all costs prior to the recent sharp rise in energy costs. However, this rise has produced a profound effect on industry and indiscriminate use of energy is being curtailed, not only through effective insulation but also through rational deployment. It would appear, therefore, that if, through better understanding of the process, hot cropping is made more efficient in terms of thermal energy consumed, a considerable saving will ensue. Apart from the running costs, hot cropping also needs extensive capital equipment and a large amount of floor space. Thus, its total elimination, should it be possible through better cropping techniques and improved bar stock in the as-received condition, is highly desirable. The present paper reports some of the preliminary work carried out to gain an understanding of the relevant parameters involved in stress cracking of cropped billet faces. It will be useful to start the discussion by considering the effects of various factors on the billet quality, starting right at the level of material composition. Factors affecting cropped billet quality The quality of cropped billets is assessed in two ways: the geometrical distortions as measured in either absolute or relative terms and the metallurgical defects as expressed in terms of work-hardening or residual stresses. In extreme cases, the metallurgical faults appear in the form of visual cracks on the cropped faces. The metallurgical defects are related to the geometric faults in the sense that large distortions usually accompany severe metallurgical faults, depending on the stress--strain characteristics of the stock material. Metallurgical defects are further affected b y the fracture process involved in cropping, as determined b y the operating conditions. Therefore, in general the quality of the cropped billet is affected b y t w o groups of parameters, namely, metallurgical and mechanical, as shown in Table 1. The former includes chemical composition, strength (hardness), micro-structure, fracture characteristics, impurities and the method of manu-
220
TABLE 1 Factors affecting the quality of cropped billets Group
Parameters
Main influence
Composition
Mainly C% and alloy content in determining strength and fracture characteristics.
Strength (hardness)
Less billet distortions in high-strength materials but affects cold shearability at very high tensile strengths.
Microstructure and segregation
Grain size and inclusions affecting the quality of the cropped faces (usually worse with larger-size bars).
Fracture charactenst~s
Brittle/ductile transition etc. resulting in stress cracking at ambient temperatures.
Impurities
Hydrogen embrittlement of cropped faces.
Method of manufacture
Rate of cooling affecting above physical properties and impurities.
Cross-sectionai area
High loads, leading to distortions and residual stresses etc.
Bar face orientation
Less geometric distortions with increased sectional rigidity (e.g. diagonal cropping of square billets).
Specific billet length or the aspect ratio (L/D)
Billet quality worsens as the aspect ratio drops.
Capacity
Near stalling situation leading to billet defects in low-capacity machine.
Tool stroke
Double-shear or "follow-through" defects with unnecessary long stroke.
Speed
Less distortions and cleaner crops at high speeds.
Rigidity
Affects tool clearances etc.
Blade design
Type of blades (e.g. " o p e n " type blade produces inferior crops as compared to "closed" type blades).
Cutting edge
Dull edges cause delay in fracture initiation resulting in high distortion.
Bar-holding (clamping)
Reduces distortions and variation in billet weight.
Off-cut support
Reduces bending and improves billet quality.
Aligmnent and clearances
Optimised conditions for a material and bar size yield better billets.
Metallurgical Material
Mechanical Billet geometry
Machine
Tooling
221
facture involved in the primary process of rolling. The mechanical group of parameters can, further, be divided into three sub-groups: billet geometry, machine, and tooling characteristics. The right-hand side of Table 1 indicates the main influence of these parameters as reflected in the quality of the cropped billets. These are discussed below in more detail with particular reference to their role in stress cracking of the cropped faces. Metallurgical parameters
(a) Chemical composition: The chemical composition of the bar stock has great influence on the quality of the cropped billets, and is a critical factor in deciding the cold cropability of the material using conventional cropping methods. A survey of the current practice referred to b y Marston [1] shows that most carbon steels can be cropped cold, provided that the carbon content does n o t exceed 0.5%. On the basis of these findings, Marston was able to divide the majority of En series and other steels into two groups: (i) material which may be cold cropped and (ii) material which requires preheating, as shown in Table 2. The suggested temperature range for obtaining satisfactory crops in most steels on the right-hand side in Table 2 is between 300 and 400°C. The importance of the carbon content in deciding whether a steel should be cold or hot cropped can be seen from Figs. 2, 3 and 4. In all cases, square 6
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Fig. 2. E f f e c t o f Ni a n d C c o n t e n t o n t h e need t o p r e h e a t E n series steels for crack-free billets.
222
TABLE 2
Division of materials f o r c o l d a n d h o t c r o p p i n g ( a f t e r Ref. 1) Material which may b e c r o p p e d cold
Material which requires preheating
En 1 En 2 En 3 En 5 En 8 E n 14 E n 16 u p t o 2.25 in 2 E n 18 u p t o 2.5 in 2 E n 24 u p t o 1.75 in 2 E n 32 E n 35 E n 36 u p t o 2.75 in 2 E n 39 u p t o 2 in 2 En 43 E n 1 1 0 u p t o 2.5 in 2 En 201 E n 351 En 352 E n 3 5 3 u p t o 2.5 in ~ En 354 En 361 En 362 S.A.E. 1 0 4 2 S.A.E. 1 0 4 6 S.A.E. 8 6 1 5 S.A.E. 8 6 2 0 S.A.E. 8 6 2 2 S.A.E. 4 6 2 0
En 9 E n 17 E n 18 a b o v e 2.5 in: E n 19 E n 24 a b o v e 1.75 in s En 26 E n 39 a b o v e 2 in s E n 40 En 100 E n 100 a b o v e 2.5 in 2 E n 353 a b o v e 2.5 in 2 S.A.E. 8 6 4 0 S.A.E. 4 1 4 0
points indicate the hot-cropped steels whereas circles represent the cold-cropped steels. It is interesting to note that the hot-cropped steels contain more than 0.3% C and lie on the right-hand side of the line AB (Figs. 3 and 4). An examination of the effect of the content of the alloying elements on the need to preheat did n o t show any specific pattern. The only exception was nickel content, as can be seen from the graph plotted in Fig. 2. Some of the steels with a high nickel content have a carbon content of less than 0.4%, b u t need preheating to avoid stress cracking*. It m a y be added that some forgers have reported stress cracking with steels having a high manganese content (over 1% Mn). * T h e o n l y e x c e p t i o n in Fig. 2 is E n 3 5 3 w h i c h n e e d s p r e h e a t i n g d e s p i t e its l o w c a r b o n a n d n i c k e l content. This is due to the presence o f banded segregation in the m i c r o - s t r u c t u r e o f t h e m a t e r i a l , as e x p l a i n e d in Ref. 3.
223
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(b) Material strength (hardness): In general, neither very soft nor very hard materials respond favourably to cropping. The former produces large billet distortions while the latter tends to increase the metallurgical defects. Following the work reported by Marston [ 1 ], who drew the line of hot cropping at a UTS of about 770 N/mm 2 (50 tonf/in2), Thomas [2] conducted a comprehensive survey to assist drop-forgers in deciding which material will require preheating. This was attempted by plotting data covering a range of UTS as a function of carbon content, in order to divide steels on the basis of UTS and carbon content. However, as shown by the insert to Fig. 3, this graph showed considerable scatter and no clear indication of UTS effect on the need to preheat could be obtained. When more data on En series and other steels were added, the scatter appeared even more pronounced with respect to the assumed boundaries, as can be seen from Fig. 3. However, a dividing line (AB) can easily be drawn on the basis of carbon content alone. Therefore, the indications are that UTS is not a criterion on which division can be made for cold and hot cropping of steels.
224
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( c ) Micro-structure and segregation: These parameters essentially provide the history of rolling and subsequent cooling as employed in the primary process of bar stock manufacture. By microscopic examination of the material, certain conclusions can be drawn with regard to the cold cropability of the material. In certain cropped specimens of En 353 exhibiting stress cracking, the reason for the defect was found to be in the micro-structure [3], where segregation was more pronounced towards the billet axis. A closer examination of a section through the axis of the specimen showed that the stress crack had progressively jumped from one bainite band to another. Axial segregation of another type, i.e. a non-metallic inclusion stringer, has also been found to be responsible for triggering stress cracking in certain cases [3]. As discussed later in connection with size effects, the grain-structure and inclusions can be significantly different in sections of different sizes, although nominally the same chemical composition is maintained (see Figs. 7 and 8). It must be added that microscopic examination has limitations, as discussed by Wannel [4]. The primary disadvantage is the total volume of material
225
actually surveyed, which is still comparatively small even with the aid of a modern quantitative microscope covering a large number of fields. Further, it takes a considerable time to prepare specimens and the results are prone to subjective judgements. However, this kind of study can be used for qualitative evaluation. (d) Fracture characteristics: The problem of stress cracking is more pronounced during winter months and in practice (Continental and American forgers) the preheating temperature achieved through steam or boiling water can be as low as 80--90°C. Steel suppliers often warn their customers to ensure that during winter the stock is stored indoors for a suitable length of time prior to cropping [4]. It is unlikely that such temperature changes will bring a b o u t significant tensile property variation. However, such a temperature change can cause a considerable alteration in impact properties, as pointed out by Thomas [2] in connection with a study of the problems associated with stress cracking. Thomas' reasoning was that if low preheating temperatures are effective in improving shearing characteristics, there should be some correlation between shearing behaviour and impact strength. To test this hypothesis, he plotted the Izod impact values and the carbon contents for a few steels. As shown in the insert to Fig. 4, this led to the conclusion that an Izod value of 40 ft-lbf (54 Joules) is required for safe cold cropping. However, when a lot more data are added, as shown in Fig. 4, there is no correlation between the impact strength and hot cropping of steels, as a number of steels with an Izod impact strength lower than 40 ft =lbf can be cold cropped and vice versa. In fact, the dominant parameter seems to be the carbon content (greater than 0.3%) and as is clearly seen in Fig. 4, the steels on the right-hand side of the line AB require preheating. The negative conclusion drawn from the plotting of Izod impact strength of steel, however, does not disprove the hypothesis that impact properties play an important role in stress cracking. As will be discussed later, in connection with the effects of preheating, the brittle/ductile transition curves of steels indicate that if preheating temperature is raised well b e y o n d the transition temperature, stress cracking is eliminated [5]. (e) Impurities: The most serious impurity, with reference to stress cracking of cropped billets, is the presence of hydrogen. High-sulphur steels are also prone to this defect [2]. The role played b y hydrogen content and hardness has been investigated b y the British Steel Corporation [4] through monitoring the hydrogen present in original casts and subsequent cold cropped billet quality as reported b y users. Fig. 5 shows the relationship between original hydrogen content in the steel casts and the as-rolled hardness, in identifying the crack susceptibility. The results presented in Fig. 5 confirm that hydrogen embrittlement resulting in the stress cracking of billet ends is more likely to occur with high hardness (therefore, high strength). From this study, it was concluded that appropriate cooling procedure should be employed after rolling in order to reduce the amount of hydrogen, for improving the cold shearability of steels. This aspect of the problem is further discussed in the
226
300 280
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G EXPECTED 220
IuJ
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N
NO CRACKING
180 160
N 0
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6
Fig. 5. Relationship between hydrogen content and delayed crop cracking (after Ref. 4).
next section while discussing the role of the method of manufacture of the bar stock. The effect of the hardness {strength) in increasing the susceptibility of hydrogen embrittlement is also supported by the analogous finding relating to the failure of electro-plated fasteners, especially of high tensile bolts (tensile strength greater than 1000 N/mm ~ or 65 tonf/in ~) [6]. The risk of failure by delayed cracking as a result of hydrogen embrittlement was found to increase with material hardness (or strength) as can be seen from the results presented in Fig. 6. Admittedly, in this case the hydrogen is artificially induced during the electro-plating process itself. (f) Method of manufacture: This relates to the influence of the controlling parameters during the process of rolling and the thermal history immediately thereafter. In the present context, controlled cooling is required to allow the hydrogen to diffuse [4], especially in large sections of steels with high alloy
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Fig. 6. Increased risk of hydrogen embrittlement at higher strengths (after Ref. 6).
227
content. It is k n o w n that hydrogen removal is most effective at a temperature just below the transformation temperature (around 600--650°C) in ferritic steels. This can be achieved b y employing a slow rate of cooling through the use of a pit immediately on completion of rolling. For this reason, it is a common practice amongst forgers to order steels for cropping in the pit-cooled condition. However, it should be noted that pit-cooled material need not necessarily be in a softer condition [2], since the hardness depends on the temperature at which the steel left the rolling mill table and it is not uncommon for steels to have undergone some degree of hardening. Further, pitcooled materials are, as expected, slightly more expensive than bed-cooled steels, thc~ugh they are nowhere near the cost of annealed stock. As far as the presence of defects (such as segregation, piping, surface faults etc.) is concerned, the rolling practice is regarded as primarily responsible. The quality of the original cast, the reductions given in each rolling pass, and the number of passes given between turning the stock, all influence the final product and the faults are significantly more pronounced for larger crosssections due to the relatively smaller number of passes. Mechanical parameters
(i) Billet geometry: As listed in Table 1, the important parameters determining the billet quality in this sub-group are the cross-sectional area, the bar face orientation with respect to the cropping direction, and the specific billet length or aspect ratio (L/D). The effect of the cross-sectional area or billet size is experienced in two ways. Firstly, although the gross cropping load increases linearly with area, due to the nature of the pressure distribution on the bar-tool contact lengths, the generated stress field near the cropping plane is more diffused for larger cross-sections. This results in an inferior fracture surface with relatively higher out-of-flatness (concavity) [7]. Secondly, as discussed earlier, the quality of the bar stock could be relatively worse for larger cross-sections in the asreceived condition, even though the composition is nominally the same. A typical example of this was found in specimens of En 43 C for square bars of different cross-sections when t h e y were examined microscopically. The larger size bar -- of 4 in (101.6 mm) side -- was cut into a grid of sixteen squares in order to observe whether the micro-structure and the inclusions were uniform over the whole cross-section. Incidentally, the piece of the bar under observation was taken from a sample showing stress cracks in the cropped billet ends. Fig. 7 (a) and (b) show the micro-structure and the inclusions of the sixteen squares grid, respectively. The specimens were polished and surface treated with Nital for the former. It is interesting to note that, as shown in Fig. 7 (a), the grain size was coarser in approaching the centre of the bar, indicating a lack of homogeneity. As far as the inclusions are concerned, as indicated in Fig. 7 (b), the particle sizes were considerably larger and more densely populated around the bar axis. Both of these factors are undesirable as they contribute to crack susceptibility.
228
Left-half of specimen Fig. 7. M i c r o s c o p i c e x a m i n a t i o n o f E n C steel o f 2 in x 2 in ( 5 0 . 8 m m × section.
50.8 r a m ) cross-
229
Right-half of specimen
230
Left-half of specimen Fig. 7 (continued).
231
Right-half of specimen
232
Similar microscopic examination of the smaller size bar -- of 1.5 in {38.1 mm) side -- of En 43 C showed t h a t the section was homogeneous with regard to both micro-structure and inclusions. Further, as shown in Fig. 8 (a) and (b), the grain size was very fine and the inclusions were barely visible with a very sparse distribution.
Fig. 8. (a) m i c r o - s t r u c t u r e a n d ( b ) i n c l u s i o n s o f E n 4 3 C o f 1.5 in x 1.5 in ( 3 8 . 1 m m x 38.1 m m ) cross-section.
233
Although the results discussed above are based on experience with En 43 C, the trend is likely to be true for most steels which presently require preheating. This is further supported by the fact that there is a recognised "size effect", in so far as most of the preheating requirements are only for larger crosssections (usually above 2.5 in side [1,2] and in some cases larger than 4 in side square bars [8] ). As far as the bar face orientation in relation to the cropping direction is concerned, this is only relevant for square bar, which is the t y p e of section most frequently used b y forgers. The practice of using V-blades for cropping in the diagonal direction is widespread. A recent study [9] has shown that the billets produced b y V-blades are superior to those produced b y flat blades, in that far less distortion is produced. It is assumed that this results from an increase in the load carrying capacity of the section as well as from the gradual deceleration of the tool (gradual loading) when the section is diagonally disposed. However, the depression on the cropped face or the outof-flatness tends to be slightly higher in diagonal cropping than in flat cropping
[lO]. The effect of the specific billet length or the aspect ratio (L/D) on the billet quality is well known. As long as the aspect ratio does not fall below a certain value (LID = 1.5 approx, in conventional single cropping), its influence is not serious. However, with decreased aspect ratio i.e. when a relatively shorter length of billet is cropped, the billet quality deteriorates rapidly. Apart from geometrical distortions, the degree of out-of-flatness or concavity of the cropped face becomes increasingly more marked. Thus, the crack susceptibility of the end faces should increase if relatively shorter billets are cropped. With the comparatively higher prices for lower size steel bars, there is a tendency to use larger sizes with a corresponding reduction in billet length. However, this practice could accelerate the cracking problem, unless of course an improved cropping technique -- such as double cropping -- is employed, which allows the use of an aspect ratio as low as 0.4 without any marked deterioration in billet quality [7 ]. (ii) Machine parameters: Mechanical presses are customarily used b y forgers for the production of cropped billets. The capacity of a cropping machine is primarily defined b y the maximum number of useful strokes per minute it can deliver at the maximum section size of the bar stock. Obviously, the maximum size of the bar stock will depend on the strength of the material. Thomas [2] has produced some useful nomographs showing the relationship b e t w e e n strength and maximum size shearable for different materials. In the context of stress cracking in billet ends, it is important to avoid stalling, or any sharp fall in the cropping speed, as this can lead to the tearing mode of failure of the unsheared central portion of the bar cross-section, which is the area most sensitive to stress cracking. Since the problem of stress cracking is almost non-existent for smaller size bars, there is a growing suspicion that uniform speed of cropping is partly responsible for good crops. It is worth mentioning that the tool stroke does not adversely affect the billet quality unless the travel is too long, in which case it produces a char-
234
acteristic "double shear" or "follow-through" defect. As far as the influence of the cropping speed is concerned, an increase in speed improves the billet quality in all respects. However, mechanical croppers have a limited capacity for increase of speed by any appreciable degree. Recent work [11] on high speed cropping (above 5 m/s) has shown that it is possible to obtain billets of excellent geometric shape, maintaining close weight tolerances and minimal metallurgical defects. High speed cropping, it is suggested, is also likely to eliminate stress cracking. However, the hardware presently available is only suitable for round bars of limited cros~sections (within 3.5 in -- 88.9 mm -- diameter). A cropping machine is supposed to be rigid enough to prevent undue deflections, as these can cause dynamic alteration of the set parameters such as blade clearance. With very large size bars involving high cropping loads, machine rigidity can be a problem. There are as yet no data available in the literature on the effect of machine flexibility on billet quality and so its influence on stress cracking cannot be assessed. (iii) Tooling parameters: The most important among these parameters is blade design and the provisions for bar and billet support during cropping. Optimum performance is affected by the condition of the cutting edge, its alignment, and its clearance setting, but these are essentially controllable. Guidance on cropping blades and machine settings is usually supplied by the machine manufacturers and is also available in the literature. The design of the cropping blade is dependent on the type of machine and the way the moving blade holder is arranged. In self-contained toolsets, the moving blade holder is not attached to the moving assembly of the press and in such cases a "closed'. type blade is compatible with feeding requirements, producing superior quality billets [11]. However, the cropping of square bars in mechanical presses requires a reciprocating moving blade carrier permanently attached to the moving assembly. In such cases, the moving blade design is usually in the shape of an inverted V for cropping the bar in the diagonal direction. This arrangement provides a half enclosure for the off-cut but at the fixed blade side there can be bar holding with efficient resistance to bending. Some shearing machines are supplied with an upholder to provide additional billet support, but forgers tend to ignore this as it limits the available energy, thereby reducing the size of the section that can be cropped by about 20% [2]. However, an upholder should be used as it improves the billet quality and thus reduces susceptibility to stress cracking. No comparable data are available showing the effectiveness of upholders in relation to stress cracking in cropped billet ends. Effect of preheating As discussed earlier, the purpose of preheating the bar stock is to reduce or eliminate the chances of stress cracking in cropped billet ends. The implication is that preheating must influence the properties of bar stock in a manner
235
conducive to the purpose. In a parallel, though not analogous, process (warm forging), the bar stock is heated to within the temperature range 300--800°C with the aim of creating lower loads and energies than are usual with cold forging. A second aim is to eliminate the need for special billet preparation such as phosphating [12]. In the case of warm/hot cropping, however, the reduction of load and energy is n o t of primary importance although it certainly contributes to the maintenance of a relatively uniform speed. An early -- 1902 -- reference on hot cropping can be found in the work of Codron [13] who observed an increase in material ductility leading to an improvement in billet quality when the bar stock was heated to 1000°C. In particular, the penetration depth was increased with a corresponding reduction of the fractured zone. Cropping tests [3] conducted within the temperature range 80--300°C on 2.5 in square bars of En 353, showed that these preheat temperatures produce less distortions in shear planes. As shown in Fig. 9, when the preheat temperature was progressively increased, the degree of outof-flatness of the cropped faces improved: that is, the depression suffered b y the concave face became lesser in magnitude. The above finding is significant for the stress cracking aspect of billet quality. Cropping generates two surfaces, one on the bar end side which is largely concave and the other on the off-cut side which is (matching) convex. It is rare to find both these surfaces exhibiting stress cracks and almost invariably, it is the concave face which is prone to stress cracking. Fig. 10 shows the opposite cropped surfaces with a typical diagonal crack running through the concave face [ 3]. Thus, the effectiveness of preheating partly
MATERIAL
En 353,
SIZE: 2.5 in sq.
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PREHEAT TEMPERATURE
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236
L~
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Fig. 10. Opposite cropped surfaces with a typical diagonal crack running on the concave face (after Ref. 3). lies in i m p r o v i n g t h e q u a l i t y o f t h e c o n c a v e face t h e r e b y i n h i b i t i n g t h e c r a c k growth. A c c o r d i n g t o C a r t w r i g h t [ 5 ] , t h e r e a s o n w h y t h e c o n c a v e s h e a r f a c e is t h e o n e t h a t cracks is t h e p r e s e n c e o f relatively higher residual tensile stresses a t
237
the centre of the face. He found support in Scheuermann's [10] results on residual stresses showing 20% more residual stress in the concave end as compared to the corresponding convex face. Further, Scheuermann concluded t h a t residual stresses in the vicinity of the cropped face decrease with the rise in preheat temperature. Some of the residual stress values experienced a drop of about 50% between room temperature and 450°C. Even for a low temperature case when the bar stock was heated in a hot water bath, a small reduction of the residual stresses was found. However, Scheuermann's results should be treated with caution as the method employed for the measurement (strain gauge rosettes) is not entirely satisfactorily. Also, for accurate determination of the residual stresses, the cropped faces should be as flat as possible, a requirement which c a n n o t be met in practice. The Drop Forging Research Association's research programme [5] on stress cracking included the compilation of data on the impact properties and relative brittleness of the bar stock over a given temperature range. The intention was that the data collected might be used to predict minimum preheat requirements and also to develop simple room temperature test sequences for use by drop forgers to identify risk of stress cracking. Although it remains to be seen whether such definitive answers to the problem can be found, certain interesting results have emerged, such as the relationship o f impact strength and brittleness to the temperature of the stock and the occurrence of stress cracks, as shown in Fig. l l ( a ) and (b) for En 353 and En 43 B steels. Both
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280
238
these steels are prone to stress cracking in the cold crop condition. At temperatures b e y o n d the brittle/ductile transition range, the specimens tested were free from stress cracking, the implication being that the preheating temperature must be well above the transition temperature. One more conclusion has been derived from the D F R A work: the fracture appearance of bar stock provides a better indication of crack risk than energy absorption, the shape and slope of the curves being very uniform. Lastly, there have been some reports of Russian work [14] on h o t cropping of high strength steels, in connection with very short billet cropping under axial compressive stresses. The cropped faces, which showed micro-cracks for the cold condition, were free from cracks when the temperature of the bar stock was raised to 400--700°C. This was attributed to an increase in material ductility, and a drop in b o t h the cropping load and the required axial compressive stresses was also observed with the rise in temperature. Conclusions
From the present study, it appears that as far as the stress cracking of billet ends is concerned, metallurgical parameters are as important as mechanical parameters. Metallurgical parameters can be controlled through choice of rolling technique, subsequent thermal treatment, and a reduction of hydrogen c o n t e n t either in the original cast and/or b y subsequent diffusion. The control of mechanical parameters involves choice in alignment, cropping speed, billet support etc., all of which determine the degree of distortion. D o u b l e cropping is an example of controlling certain mechanical parameters, such as bar and billet support, yielding better crops. Fig. 12 shows typical improvements in billet quality from double cropping when compared with the traditional m e t h o d of single cropping. A crucial factor is concavity depth which, if reduced, should reduce the risk of stress cracking. As shown in Fig. 12 this factor is reduced by half for the case of double cropping. Further, the strainhardening is considerably reduced as evidenced from the plotting of iso-hardness lines in Fig. 12, representing the relative rise over the base hardness of the material. Combination of these t w o factors ought to minimise the chance of stress cracking. Improvement can also be gained through the use of high cropping speeds. Cropping machines should n o t be used at the t o p end of their capacities and preferably an over-rated machine should be chosen, as the changes in cropping speed will be smaller. On the question of more efficient use of thermal energy, there is, as yet, no rational basis other than the trial-and-error approach, though it is fairly well established that high-risk steels can be identified on the basis of carbon content. High-strength steels are more prone to stress cracking simply because hydogen embrittlement is more pronounced in them, even at minute levels.
239
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Acknowledgements The work reported here forms a part of a comprehensive programme of research on "Metal Forming and Associated Areas" supported b y a Consolidated Research Grant from the Science Research Council (U.K.), and under the general guidance of Professor S.A. Tobias, to w h o m the authors are thankful.
References 1 G.V. Marston, Met. Treat. Drop Forg., November (1963) 437. 2 A. Thomas, Metall. Met. Form., 41 (1974) 198. 3 Anon., D F R A Report No. RC 75/145, Drop Forging Research Association, Sheffield, 1975. 4 P.H. Wannel, Metall. Met. Form., 43 (1976) 207. 5 B. Cartwright, Delayed crop cracking, to be published. 6 D.G. James, Plating News, 3M Publication, April (1978) 6. 7 M.H.M. Ahmed, A study of the multiple cropping process, Ph.D. Thesis, University of Birmingham, 1978. 8 W.C. Tucker, Steel Process., November (1954) 695. 9 M.H.M. Ahmed and M.K. Das, An experimental study of the double cropping process, to be presented at the 19th MTDR Conf., Manchester, September 1978. 10 H. Scheuermann, Ind. Anz., 97 (1975) 1538. 11 M.K. Das and S.A. Tobias, Metall. Met. Form., 43 (1976) 47. 12 S.E. Rogers, Metall. Met. Fo~m., 43 (1976) 36. 13 C. Codron, Experiments on the work of the machines used foo' metals (in French), Vols. I and II, Dunod et Pinat, Paris, 1902 and 1906. 14 G. Tcemann, Povyshenie Tochnosti i automatizatsiya shtampovki i kovki, Book No. 9, edited by V.T. Mesherin, Moscow, 1971.