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Manganese steel for abrasive environments
Manganese steel for abrasive environments A conditioning process for Hadfield's manganese steel and a novel method of producing FAM bearings from the same material D. Michalon, G. Mazet & Ch. Burgio* Hadfield's manganese steel, containing approximately 1.2% carbon and 1 2 14% manganese, is almost exclusively used in industry in the austenitic condition (Fig 1). Austenitic structures have poor wear resistance, and are therefore seldom used in applications involving sliding or rolling. It is known, however, that cold working, which occurs under certain operating conditions, can bring about local transition of austenite to martensite. High manganese steel is found to acquire excellent resistance to wear and abrasion after a certain time of operation. Unfortunately before this desirable condition is attained some preliminary wear of the austenitic surface takes place, and this restricts the field of industrial applications for this steel. The amount of preliminary wear varies according to the operating conditions, which govern the amount of cold work taking place. Thus if the rate of wear is greater than the rate of work hardening the components will deteriorate rapidly. Under more favourable conditions, when work hardening is occurring more rapidly than wear, the rate of wear or deterioration decreases progressively as the time in operation increases. Various means are currently used in industry to reduce the amount of untransformed austenite present at the surface of parts before they are put into service. The preliminary wear which can occur before this austenite is transformed to martensite by cold working in service militates against efficient operation of the parts. Shot peening, tempering, rolling, and roll burnishing are examples of such 'preconditioning' treatments, but it is generally admitted that none is completely satisfactory.
* HEF, Andrezioux Boutheon 42160, France. Translated by J. C. Gregory t FinancialaM from DGRST, France
Other methods of solving this problem of initial wear of manganese steels used in frictional situations have been investigated, and this article describes a solution involving a combination of mechanical and thermal surface treatmentst. A somewhat different approach to the problem which applies to plain bearings used in abrasive situations is also described.
the precipitation of iron and manganese carbides at the grain boundaries, progressively followed by the appearance of an acicular constituent which later extends to the interior of the grains, and by the formation of nodular troostite, also at the grain boundaries (Figs 2 and 3). The reprecipitation of carbides at the grain boundaries in this way results in a loss of toughness and a return to a brittle condition. This has been shown in tests on a simulator.
Simulator used for testing
The microstructure of this steel is unusual. It is normally used in industry in the heat treated condition involving heating to 1050°C and cooling quickly, eg by water quenching. The original
Hardness tests and microscopic examination of samples have been used to study the effectiveness of various operations and parameters in breaking down the austenite in the superficial layers of water toughened manganese steel. A more positive test has been devised by the construction of a simulator (Fig 4) to simulate conditions in what are called in France 'articulations' ie couplings or linkages. Two shafts oscillate within two bearings. The shafts a r e driven by the motion of a hydraulic jack through a rack and pinion system. A further jack with two pistons allows a variable load to be applied to the shaft/bearing test rigs.
Fig 1 Microstructure o f HadfieM's manganese steel (Z 120 M 12) water quenched from 1050°C, showing austenite grains (500 x)
Fig 2 Effect o f tempering manganese steel previously in the austenitic condition, showing precipitation o f carbides at the grain boundaries (800 x )
Structure of Hadfield's manganese steel
manufacturers of the steel called this treatment 'water toughening'. It results in the solid solution of carbides causing brittleness and the production of almost pure austenite. The austenite grain boundaries are well defined and of approximately uniform thickness (Fig 1). The hardness in this condition is of the order of 250 HV, depending on the load used for testing. If water toughened manganese steel is tempered, partial decomposition of the austenite occurs. The extent of this decomposition depends on the time and temperature of the tempering treatment. This begins immediately by
Fig 3 Effect or further tempering manganese steel in the austenitic condition. Acicular precipitates (martensite) extending within the grains and some nodular troostite at the grain boundaries (800 x )
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The whole assembly can be totally enclosed to allow testing to be carried out in various environments, such as when immersed in water or slurry or in an atmosphere contaminated with dust in suspension.
•~Shaft
jHydraulic
[~,,uu~,~_ @
~
~Bearing
~'~]~'~
Raek
In this investigation testing conditions were standardised: Load: 200 bars Speed: 0.05 m/s Amplitude of shaft oscillations: _+90° Frequency of shaft oscillations: 1 Hz
Environment: Testpieces immersed in a slurry of water, silica, and clay Lubrication: Testpieces smeared with a film of EP grease on assembly. No further lubricant supplied
Preliminary selection of preconditioning treatments To assist in the ultimate development of an effective preconditioning treatment, simulator tests have been carried out on manganese steel shafts and bearings in three different conditions: 'Normal' condition - water toughened by quenching from 1050°C: in every case the tests ended within the first hour due to the incidence of scuffing and seizure (Figs 5 and 6).
Fig 4
Simulator for friction tests
Fig 5 Watertoughened manganese steel shaft from simulator after less than 1 h in operation
Water toughened material tempered at 580°C for 3 h: six tests were started but three had to be discontinued following breakage of the shafts due to excessive brittleness. The remaining three were stopped after running for 90 h, when wear of the order of 1 mm had taken place. The condition of the tested surfaces is shown in Figs 7 and 8.
Fig 6 Watertoughened manganese steel beeHng from simulator after less than 1 h in operation
172 TRIBOLOGYinternationalAugust1976
Careful examination of the testpieces shows the wear and tear to be due to three main factors: surface phenomena such as scuffing and wear; effects due to fatigue occuring at positions below the surface, such as the production of cracks in zones of maximum shear stress; and the low toughness o f shafts which had been given a tempering treatment.
HEF preconditioning process In determing the best way of achieving the desired preconditioning o f the surfaces o f Hadfield's manganese steel parts, attempts have been made to remedy the factors leading to failure, while endeavouring to meet the following requirements: 1 To accomplish a large degree o f transformation of the austenite, by mechanical means, for a distance of 0.2 mm from the surface. This transformation should be sufficiently complete to avoid the possibility of further decomposition if subsequently externally heated, for example, to 600°C. It has been shown that structures produced as a result of the breakdown of austenite by cold working have a good wear resistance, while those produced by tempering are less useful for this purpose.
Fig 7 Water toughened manganese steel shaft, tempered for 3 h at 580°C and r u n in simulator for 90 h
2 To confer a hardness of at least 700 HV up to a minimum depth of 0.2 mm from the surface. This hardness should then fall progressively to equal that of the core at 1.5 2.0 mm from the surface. It is essential that a hardness of more than 500 HV should exist at 0.5 mm depth in order to prevent the incidence of shearing beneath the surface. -
Fig 8 Water toughened manganese steel bearing, tempered for 3 h at 580° C and run in simulator for 90 h
Water toughened material cold worked by roll burnishing: four tests were carried out on shafts and bearings which had been lightly cold worked by roll burnishing, so as to give a hardness of the order of 600 HV up to a depth of 50 microns, and where the hardness falls to that of the core at 0.5 mm from the surface (Fig 9). None o f the four tests ran for more than 300 h because appreciable wear had occurred (Figs 10 and 11).
60C
o~5OC 40C
30C
IOO
200
300
400
500
IOOO
DlSTonce f r o m s u t f o c e , m i c r o n s
Fig 9 Depth~hardness curve for lightly cold worked manganese steel testpieces.
3 To introduce a pattern o f machine marks running largely at right angles to the direction of movement of the surface in use. This requirement, which has been explained elsewhere by one of the authors, is necessary to ensure that sliding occurs preferentially by shearing at the couple interface and not b y welding and subsequent tearing o f the weld junctions. These machine marks or grooves also facilitate the escape of wear debris and the retention of reserves of lubricant. 4 To produce a surface layer on manganese steel components having the
TRIBOLOGY international August 1976 173
800 700 -° ~6°° 500
~
400
300 200 )00
200 300 400 500 D,stancefrom surface,microns
J I000
Fig 12 Depth/hardness curve of water toughened manganese steel after knurling and roll burnishing 700 3 -- 600 2 o
3 Load 700 do N 2 --_L O O d a ~do 6 0N0 - -
1->500 400 300 200
Lightly cold worked shaft after running for less than 300 h
~o
~b6" ~ b
400
500
dooo
Dtstoncefrom surfoce,m,crons
Fig 13 Effect of the magnitude of the load used in the knurling operation on the depth/hardness curve
Cold working by knurling This operation produces the required hardness gradient (Fig 12), and also the required pattern of machine marks or grooves. Most o f the austenite present in the superficial layers is decomposed. The degree o f deformation allowed was standardised while varying the two well know parameters encountered in cold working operations - applied load and rate o f deformation. The total time during which cold work was applied was also varied. While seldom referred to in the literature, this parameter had an important influence on the results o f these tests. Details of the tests carried out to determine the effect of these three variables and the results obtained are described below:
Fig l l
Lightly coM worked bearing after running for less than 300 h
property of inhibiting frictional welding with the aim of avoiding scuffing in use. This is necessary because whatever mechanical or thermal treatments are applied to this steel, it is never possible to eliminate entirely retained austenite from the microstructure, and it is this austenite which gives the risk of scuffing. To avoid completely any further machining operations after work
174
hardening b y mechanical means. This means that during the final cold working operations attention will have to be given to control b o t h surface finish and tolerances (Class 8 in the ISO system o f classifying tolerances). It was finally decided to use the sequence of operations described below in order to effect the desired preconditioning of water toughened manganese steel.
TRIBOLOGY international August 1976
Effect of the magnitude of the load applied to the knurling tool: these tests were carried out on a horizontal lathe using loads of 700 daN, 600 daN, and 300 daN respectively. The testpieces of Hadfield's manganese steel had a diameter of 16 mm and were rotated at 100 rpm. The knurling tools were 20 mm in diameter with a 10 mm face width and a 1.5 mm pitch. Depth/ hardness tests using a 100 g load gave the results shown in Fig 13. It will be seen that the magnitude of the applied load has a marked effect on the extent
of work hardening and that beyond a certain load, in this case 600 daN, a further increase in load gives little corresponding increase in hardness. It has been noted, however, that at loads greater than 600 daN cracking appears in the surface layers. Effect of the rate of deformation as governed by the speed of rotation of the testpiece: these tests were done on a horizontal lathe by varying the speed of rotation so as to give equivalent peripheral speeds of the testpiece of 5 m/ rain, 10 m/min, 30 m/min, and 50 m/ min respectively. Depth/hardness curves were plotted using a 100 g load (Fig 14) from which a close relationship between rate of deformation and extent of work hardening can be deduced. Thus at a depth of 25 microns, when the peripheral speed of the testpiece was 50 m/min the hardness did not exceed 500 HV, whilst with a speed of only 10 m/rain the hardness was of the order of 680 HV. However, if the speed is reduced below 10 m/ min a maximum degree of work hardening appears to be reached, such that even at only half this speed little further increase in hardness results. Effect of total time during which cold work was applied: preliminary tests had shown that if a knurling tool was rotated in contact with the surface of a shaft without advancing the tool, that is, always bearing on the same portion of the shaft, the degree of work hardening was a function of the number of times the tool passed over the same point on the shaft. This discovery led to a further series of tests designed to obtain quantitative information by varying the number of 'passes' of the tool and comparing this with the hardness attained. The results are plotted in Fig 15, and confirm the original findings. It is obvious that as the number of passes is increased a maximum increase in hardness will eventually be reached, and there is evidence that the rate at which the hardness increases has slowed down appreciably between 20 and 30 passes. After 30 passes a hardness of about 730 HV was obtained at a depth of 25 microns and there appeared to be little point in going further than 30 passes.
and shafts were used which had previously been knurled as described above, ie with a load of 600 daN, a peripheral speed of 10 m/min, and applying 30 passes at the same position o n the shaft. All these tests showed that the roll burnishing operation should be carried out with a shaft driven at 10 m/min, a load of 300 daN and applying 10 passes of the burnishing tool to the shaft. The combined knurling and burnishing operations result in: a hardness of approximately 800 HV up to a depth of 0.2 mm; 0.4 mm from the surface, the hardness is still as high as 600 HV; a gradual fall in hardness to that of the core at 1.5 mm from the surface.
Low temperature sulphurising This operation produces a layer of iron sulphide at the surface of treated parts which is resistant to welding. The Sulf BT process was used. This produces a layer which is strongly adherent to the base metal by electrolytic means. The treatment was observed to have an additional advantage in that it removes by chemical attack any small burrs of heavily cold worked metal, which is very fragile, and results from the burnishing operation.
Tests on treated components Having established the preferred procedure for an effective preconditioning 70C 4 5~ '~
60( o(~
4-4-Per,pherol speed 5 m/ran 3 peripheral speed } 0 m / r a n speed I -Peripherol speed 50
2~2-peripheral
50~ I
30m/ran m/ran
400
200 t _ IO0
2OO 300 40O 50O Distance from surface, microns
IOOO
Fig 14 Effect o f the speed o f deformation in the knurling operation on the depth/hardness curve
700 3
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500I~ 400
; _ 23o° ,, Pp:::::"
1
i i2-! !IO "posses" I - 3 "posses"
Cold working by roll burnishing 200
The object of this operation is to complete the cold working of the surface and to conform to requirements of tolerance and surface finish (Class 8). Tests were again carried out on a lathe,
Running time: 1120 hours Condition of surface: Polished Wear on shaft: 0.1 mm (Fig 16) Wear on bearing: 0.2 mm (Fig 17) F o r purposes of comparison recall testpieces which had only been water toughened and not subjected to the HEF preconditioning treatment r a n for less than an hour. Those that were water-toughened and tempered to precipitate iron and manganese carbides ran for between 60 and 90 h. The advantages of the HEF treatment are equally well illustrated by comparing the appearance of the surfaces of shafts and bearings after testing. (Figs 5 - 8 , 16, 17).
Industrial operation Operation of the HEE process is simple when dealing with shafts. It is carried out as described above by a combination of knurling, roll burnishing and finally Sulf BT treatment. Fig 18 shows the type of surface obtained on an unused part by knurling and roll burnishing. It is not such a simple matter in the case of bearings and in certain cases, particularly when dealing with small thin walled bearings, the process has been found difficult to carry out on an industrial scale.
Alternative process for producing plain bearings
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treatment for manganese steel components to be used in sliding or rolling conditions, testpieces were treated and subjected to simulator tests under the same conditions as those carried out when doing the preliminary selection. Results were:
I00
200 500
400 500
~000
Distance from surface, microns
Fig 15 Effect of the time o f deformation (ie total time of knurling operation) on the depth/hardness curve
Where difficulty has been experienced in the HEF preconditioning treatment of bearings, the company have adopted a new approach by applying the principles described above in a different way. In developing this additional process it had to be remembered that it should not involve any machining operations since these would have a marked effect on the price of bearings made from Hadfield's manganese steel (Z 120 M 12 specification, containing 1.2% carbon, 12.0% manganese). The process makes use of the work hardening effect of drawing manganese steel wire through dies to produce a tape. The original wire is 4 or 5 mm diameter and it is Sulf BT treated before drawing to avoid scuffing or
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scoring between wire and dies. The speed of drawing and the load applied have been chosen so as to accomplish' the degree of work hardening required. After drawing, the tape has a surface hardness of 750 HV when tested with a 100 g load, and a core hardness of 600 HV. Fig 19 shows the change in microstructure produced by passing through the dies and shows the change in hardness due to work hardening at corresponding positions.
Fig 16 Knurled, roiled, and Sulf B T treated, water toughened manganese steel shaft after 1120 h running on simulator
The work hardened tape produced in this way is wound onto a rotating mandrel, the diameter of which depends on the required interior diameter of the finished bearing. The 'spring' thus formed is inserted into a casing or former made from cold drawn A 37 mild steel tube, without subsequent welding, and when severed from the ribbon coming from the dies this spring fiattens itself against the wall of the casing (Fig 20). The tape is anchored firmly to the casing by passing the composite bearing so produced through dies having a slightly smaller diameter than the outside diameter of the casing. This causes plastic flow of the material of tile casing and results in the filling of the instertices between adjacent turns of the tape spiral. The anchorage of the spiral is completed by spot welding the tape at each end. This method of producing composite bearings is already well established industrially, and such products are
Fig 19 Austenitic manganese steel wire, Sulf B T treated and drawn through dies to produce a tape. Mierostructure of drawn section and corresponding depth/hardness curve 800~
•
o~70Q
Fig 17 Knurled, roiled, and Sulf B T treated bearing in water toughened manga. nese steel after 1120 h running on simulator
i o
Fig 18
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x
Microhordnes~ot O lmm below surfoce
/
500 480[ I
/ /
/
/
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/
/
/ / ~ Mlcrohordness a~ centre of wlre
/
/
Surface finish o f knurled and roll burnished shaft before use 2oo~ Water tou(Jhened&tructure
176
f ~
TRIBOLOGY international August 1976
Woter toughenedond cold wor~ed struclure
being marketed under the trade name of 'FAM Bearings'.
Industrial applications Components produced in this manner have excellent scuffing, wear, and abrasion resistant properties, which the following applications to industrial problems serve to illustrate: Example 1 : Couplings for conveyors built up from metal plates. Sliding speed: 0.05 m/s Load: 1 0 0 - 3 0 0 bars Environment: Highly abrasive Temperature : Ambient
Fig 2O Composite bearings fitted with a liner in spiral form of drawn manganese steel tape
(a) Conventional designs: There are two types involving either surface hardened pins running in bronze bushes fitted with greasing facilities or sealed bearings. In both cases rapid wear occurs, particularly of the bronze bushes. (b) Designs involving use of the HEF preconditioning process: Two variations have been tested, one involving both pins and bushes of nranganese steel (Z 120 M 12) treated by the HEF process, and the other where surface hardened pins run in processed manganese steel bushes. In both cases the need for greasing was eliminated, there was increased reliability, and the life was increased by three or even five times. Example 2: Couplings used on steelworks billet transporters.
Fig 21
HEF treated manganese steel leaf spring shackles for lorries
Fig22
Conveyor coupling showing knurled finish
Sliding speed: 0.1 m/s Load: 5 0 - 2 0 0 bars Environment: Dusty Temperature: Variable from 50-250°C Lubrication: Unreliable (a) Conventional design: As in example 1, there are two types, one using a steel pin and a bronze bush with provision for greasing, and the other consisting of sealed bearings. Rapid wear of the pins and bushes occurs, and there is seizing of the sealed bearings. The average life of either type is 3 - 5 months. (b) Design involving the use of the HEF preconditioning process: This consists of a pin made from manganese steel Z 120 M 12 treated by the HEF process in combination with an FAM bearing. Again there is no need to use grease and the life is three or four times as great.
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Applications A brief list of current applications is given below. It must be borne in mind that the process is still relatively new and different applications are constantly being tested. Automobile: Leaf spring shackles, suspension couplings, lorry skip trunnions Steel Industry: Conveyor parts for rolling mills, coke oven chargingmachine parts, brake couplings Public Works: Mechanical shovel joints, jack couplings, crank pins and bushes Cement Industry: Pin and bush linkages for steel plate conveyors used in cement manufacture Glass Industry: Bearings of silica extractors, charging-machine parts Foundry: Casting machine swivel parts Agricultural Industry: Various moving parts
Fig 23 Manganese steel bush treated by the HEF process, used for disc brakes with an induction hardened carbon steel pin
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TRIBOLOGY international August 1976
* HEF, Andrezioux Boutheon 42160, France. Translated by J. C. Gregory t FinancialaM from DGRST, France
Other methods of solving this problem of initial wear of manganese steels used in frictional situations have been investigated, and this article describes a solution involving a combination of mechanical and thermal surface treatmentst. A somewhat different approach to the problem which applies to plain bearings used in abrasive situations is also described.
the precipitation of iron and manganese carbides at the grain boundaries, progressively followed by the appearance of an acicular constituent which later extends to the interior of the grains, and by the formation of nodular troostite, also at the grain boundaries (Figs 2 and 3). The reprecipitation of carbides at the grain boundaries in this way results in a loss of toughness and a return to a brittle condition. This has been shown in tests on a simulator.
Simulator used for testing
The microstructure of this steel is unusual. It is normally used in industry in the heat treated condition involving heating to 1050°C and cooling quickly, eg by water quenching. The original
Hardness tests and microscopic examination of samples have been used to study the effectiveness of various operations and parameters in breaking down the austenite in the superficial layers of water toughened manganese steel. A more positive test has been devised by the construction of a simulator (Fig 4) to simulate conditions in what are called in France 'articulations' ie couplings or linkages. Two shafts oscillate within two bearings. The shafts a r e driven by the motion of a hydraulic jack through a rack and pinion system. A further jack with two pistons allows a variable load to be applied to the shaft/bearing test rigs.
Fig 1 Microstructure o f HadfieM's manganese steel (Z 120 M 12) water quenched from 1050°C, showing austenite grains (500 x)
Fig 2 Effect o f tempering manganese steel previously in the austenitic condition, showing precipitation o f carbides at the grain boundaries (800 x )
Structure of Hadfield's manganese steel
manufacturers of the steel called this treatment 'water toughening'. It results in the solid solution of carbides causing brittleness and the production of almost pure austenite. The austenite grain boundaries are well defined and of approximately uniform thickness (Fig 1). The hardness in this condition is of the order of 250 HV, depending on the load used for testing. If water toughened manganese steel is tempered, partial decomposition of the austenite occurs. The extent of this decomposition depends on the time and temperature of the tempering treatment. This begins immediately by
Fig 3 Effect or further tempering manganese steel in the austenitic condition. Acicular precipitates (martensite) extending within the grains and some nodular troostite at the grain boundaries (800 x )
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The whole assembly can be totally enclosed to allow testing to be carried out in various environments, such as when immersed in water or slurry or in an atmosphere contaminated with dust in suspension.
•~Shaft
jHydraulic
[~,,uu~,~_ @
~
~Bearing
~'~]~'~
Raek
In this investigation testing conditions were standardised: Load: 200 bars Speed: 0.05 m/s Amplitude of shaft oscillations: _+90° Frequency of shaft oscillations: 1 Hz
Environment: Testpieces immersed in a slurry of water, silica, and clay Lubrication: Testpieces smeared with a film of EP grease on assembly. No further lubricant supplied
Preliminary selection of preconditioning treatments To assist in the ultimate development of an effective preconditioning treatment, simulator tests have been carried out on manganese steel shafts and bearings in three different conditions: 'Normal' condition - water toughened by quenching from 1050°C: in every case the tests ended within the first hour due to the incidence of scuffing and seizure (Figs 5 and 6).
Fig 4
Simulator for friction tests
Fig 5 Watertoughened manganese steel shaft from simulator after less than 1 h in operation
Water toughened material tempered at 580°C for 3 h: six tests were started but three had to be discontinued following breakage of the shafts due to excessive brittleness. The remaining three were stopped after running for 90 h, when wear of the order of 1 mm had taken place. The condition of the tested surfaces is shown in Figs 7 and 8.
Fig 6 Watertoughened manganese steel beeHng from simulator after less than 1 h in operation
172 TRIBOLOGYinternationalAugust1976
Careful examination of the testpieces shows the wear and tear to be due to three main factors: surface phenomena such as scuffing and wear; effects due to fatigue occuring at positions below the surface, such as the production of cracks in zones of maximum shear stress; and the low toughness o f shafts which had been given a tempering treatment.
HEF preconditioning process In determing the best way of achieving the desired preconditioning o f the surfaces o f Hadfield's manganese steel parts, attempts have been made to remedy the factors leading to failure, while endeavouring to meet the following requirements: 1 To accomplish a large degree o f transformation of the austenite, by mechanical means, for a distance of 0.2 mm from the surface. This transformation should be sufficiently complete to avoid the possibility of further decomposition if subsequently externally heated, for example, to 600°C. It has been shown that structures produced as a result of the breakdown of austenite by cold working have a good wear resistance, while those produced by tempering are less useful for this purpose.
Fig 7 Water toughened manganese steel shaft, tempered for 3 h at 580°C and r u n in simulator for 90 h
2 To confer a hardness of at least 700 HV up to a minimum depth of 0.2 mm from the surface. This hardness should then fall progressively to equal that of the core at 1.5 2.0 mm from the surface. It is essential that a hardness of more than 500 HV should exist at 0.5 mm depth in order to prevent the incidence of shearing beneath the surface. -
Fig 8 Water toughened manganese steel bearing, tempered for 3 h at 580° C and run in simulator for 90 h
Water toughened material cold worked by roll burnishing: four tests were carried out on shafts and bearings which had been lightly cold worked by roll burnishing, so as to give a hardness of the order of 600 HV up to a depth of 50 microns, and where the hardness falls to that of the core at 0.5 mm from the surface (Fig 9). None o f the four tests ran for more than 300 h because appreciable wear had occurred (Figs 10 and 11).
60C
o~5OC 40C
30C
IOO
200
300
400
500
IOOO
DlSTonce f r o m s u t f o c e , m i c r o n s
Fig 9 Depth~hardness curve for lightly cold worked manganese steel testpieces.
3 To introduce a pattern o f machine marks running largely at right angles to the direction of movement of the surface in use. This requirement, which has been explained elsewhere by one of the authors, is necessary to ensure that sliding occurs preferentially by shearing at the couple interface and not b y welding and subsequent tearing o f the weld junctions. These machine marks or grooves also facilitate the escape of wear debris and the retention of reserves of lubricant. 4 To produce a surface layer on manganese steel components having the
TRIBOLOGY international August 1976 173
800 700 -° ~6°° 500
~
400
300 200 )00
200 300 400 500 D,stancefrom surface,microns
J I000
Fig 12 Depth/hardness curve of water toughened manganese steel after knurling and roll burnishing 700 3 -- 600 2 o
3 Load 700 do N 2 --_L O O d a ~do 6 0N0 - -
1->500 400 300 200
Lightly cold worked shaft after running for less than 300 h
~o
~b6" ~ b
400
500
dooo
Dtstoncefrom surfoce,m,crons
Fig 13 Effect of the magnitude of the load used in the knurling operation on the depth/hardness curve
Cold working by knurling This operation produces the required hardness gradient (Fig 12), and also the required pattern of machine marks or grooves. Most o f the austenite present in the superficial layers is decomposed. The degree o f deformation allowed was standardised while varying the two well know parameters encountered in cold working operations - applied load and rate o f deformation. The total time during which cold work was applied was also varied. While seldom referred to in the literature, this parameter had an important influence on the results o f these tests. Details of the tests carried out to determine the effect of these three variables and the results obtained are described below:
Fig l l
Lightly coM worked bearing after running for less than 300 h
property of inhibiting frictional welding with the aim of avoiding scuffing in use. This is necessary because whatever mechanical or thermal treatments are applied to this steel, it is never possible to eliminate entirely retained austenite from the microstructure, and it is this austenite which gives the risk of scuffing. To avoid completely any further machining operations after work
174
hardening b y mechanical means. This means that during the final cold working operations attention will have to be given to control b o t h surface finish and tolerances (Class 8 in the ISO system o f classifying tolerances). It was finally decided to use the sequence of operations described below in order to effect the desired preconditioning of water toughened manganese steel.
TRIBOLOGY international August 1976
Effect of the magnitude of the load applied to the knurling tool: these tests were carried out on a horizontal lathe using loads of 700 daN, 600 daN, and 300 daN respectively. The testpieces of Hadfield's manganese steel had a diameter of 16 mm and were rotated at 100 rpm. The knurling tools were 20 mm in diameter with a 10 mm face width and a 1.5 mm pitch. Depth/ hardness tests using a 100 g load gave the results shown in Fig 13. It will be seen that the magnitude of the applied load has a marked effect on the extent
of work hardening and that beyond a certain load, in this case 600 daN, a further increase in load gives little corresponding increase in hardness. It has been noted, however, that at loads greater than 600 daN cracking appears in the surface layers. Effect of the rate of deformation as governed by the speed of rotation of the testpiece: these tests were done on a horizontal lathe by varying the speed of rotation so as to give equivalent peripheral speeds of the testpiece of 5 m/ rain, 10 m/min, 30 m/min, and 50 m/ min respectively. Depth/hardness curves were plotted using a 100 g load (Fig 14) from which a close relationship between rate of deformation and extent of work hardening can be deduced. Thus at a depth of 25 microns, when the peripheral speed of the testpiece was 50 m/min the hardness did not exceed 500 HV, whilst with a speed of only 10 m/rain the hardness was of the order of 680 HV. However, if the speed is reduced below 10 m/ min a maximum degree of work hardening appears to be reached, such that even at only half this speed little further increase in hardness results. Effect of total time during which cold work was applied: preliminary tests had shown that if a knurling tool was rotated in contact with the surface of a shaft without advancing the tool, that is, always bearing on the same portion of the shaft, the degree of work hardening was a function of the number of times the tool passed over the same point on the shaft. This discovery led to a further series of tests designed to obtain quantitative information by varying the number of 'passes' of the tool and comparing this with the hardness attained. The results are plotted in Fig 15, and confirm the original findings. It is obvious that as the number of passes is increased a maximum increase in hardness will eventually be reached, and there is evidence that the rate at which the hardness increases has slowed down appreciably between 20 and 30 passes. After 30 passes a hardness of about 730 HV was obtained at a depth of 25 microns and there appeared to be little point in going further than 30 passes.
and shafts were used which had previously been knurled as described above, ie with a load of 600 daN, a peripheral speed of 10 m/min, and applying 30 passes at the same position o n the shaft. All these tests showed that the roll burnishing operation should be carried out with a shaft driven at 10 m/min, a load of 300 daN and applying 10 passes of the burnishing tool to the shaft. The combined knurling and burnishing operations result in: a hardness of approximately 800 HV up to a depth of 0.2 mm; 0.4 mm from the surface, the hardness is still as high as 600 HV; a gradual fall in hardness to that of the core at 1.5 mm from the surface.
Low temperature sulphurising This operation produces a layer of iron sulphide at the surface of treated parts which is resistant to welding. The Sulf BT process was used. This produces a layer which is strongly adherent to the base metal by electrolytic means. The treatment was observed to have an additional advantage in that it removes by chemical attack any small burrs of heavily cold worked metal, which is very fragile, and results from the burnishing operation.
Tests on treated components Having established the preferred procedure for an effective preconditioning 70C 4 5~ '~
60( o(~
4-4-Per,pherol speed 5 m/ran 3 peripheral speed } 0 m / r a n speed I -Peripherol speed 50
2~2-peripheral
50~ I
30m/ran m/ran
400
200 t _ IO0
2OO 300 40O 50O Distance from surface, microns
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Fig 14 Effect o f the speed o f deformation in the knurling operation on the depth/hardness curve
700 3
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500I~ 400
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Cold working by roll burnishing 200
The object of this operation is to complete the cold working of the surface and to conform to requirements of tolerance and surface finish (Class 8). Tests were again carried out on a lathe,
Running time: 1120 hours Condition of surface: Polished Wear on shaft: 0.1 mm (Fig 16) Wear on bearing: 0.2 mm (Fig 17) F o r purposes of comparison recall testpieces which had only been water toughened and not subjected to the HEF preconditioning treatment r a n for less than an hour. Those that were water-toughened and tempered to precipitate iron and manganese carbides ran for between 60 and 90 h. The advantages of the HEF treatment are equally well illustrated by comparing the appearance of the surfaces of shafts and bearings after testing. (Figs 5 - 8 , 16, 17).
Industrial operation Operation of the HEE process is simple when dealing with shafts. It is carried out as described above by a combination of knurling, roll burnishing and finally Sulf BT treatment. Fig 18 shows the type of surface obtained on an unused part by knurling and roll burnishing. It is not such a simple matter in the case of bearings and in certain cases, particularly when dealing with small thin walled bearings, the process has been found difficult to carry out on an industrial scale.
Alternative process for producing plain bearings
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treatment for manganese steel components to be used in sliding or rolling conditions, testpieces were treated and subjected to simulator tests under the same conditions as those carried out when doing the preliminary selection. Results were:
I00
200 500
400 500
~000
Distance from surface, microns
Fig 15 Effect of the time o f deformation (ie total time of knurling operation) on the depth/hardness curve
Where difficulty has been experienced in the HEF preconditioning treatment of bearings, the company have adopted a new approach by applying the principles described above in a different way. In developing this additional process it had to be remembered that it should not involve any machining operations since these would have a marked effect on the price of bearings made from Hadfield's manganese steel (Z 120 M 12 specification, containing 1.2% carbon, 12.0% manganese). The process makes use of the work hardening effect of drawing manganese steel wire through dies to produce a tape. The original wire is 4 or 5 mm diameter and it is Sulf BT treated before drawing to avoid scuffing or
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scoring between wire and dies. The speed of drawing and the load applied have been chosen so as to accomplish' the degree of work hardening required. After drawing, the tape has a surface hardness of 750 HV when tested with a 100 g load, and a core hardness of 600 HV. Fig 19 shows the change in microstructure produced by passing through the dies and shows the change in hardness due to work hardening at corresponding positions.
Fig 16 Knurled, roiled, and Sulf B T treated, water toughened manganese steel shaft after 1120 h running on simulator
The work hardened tape produced in this way is wound onto a rotating mandrel, the diameter of which depends on the required interior diameter of the finished bearing. The 'spring' thus formed is inserted into a casing or former made from cold drawn A 37 mild steel tube, without subsequent welding, and when severed from the ribbon coming from the dies this spring fiattens itself against the wall of the casing (Fig 20). The tape is anchored firmly to the casing by passing the composite bearing so produced through dies having a slightly smaller diameter than the outside diameter of the casing. This causes plastic flow of the material of tile casing and results in the filling of the instertices between adjacent turns of the tape spiral. The anchorage of the spiral is completed by spot welding the tape at each end. This method of producing composite bearings is already well established industrially, and such products are
Fig 19 Austenitic manganese steel wire, Sulf B T treated and drawn through dies to produce a tape. Mierostructure of drawn section and corresponding depth/hardness curve 800~
•
o~70Q
Fig 17 Knurled, roiled, and Sulf B T treated bearing in water toughened manga. nese steel after 1120 h running on simulator
i o
Fig 18
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x
Microhordnes~ot O lmm below surfoce
/
500 480[ I
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/
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/ / ~ Mlcrohordness a~ centre of wlre
/
/
Surface finish o f knurled and roll burnished shaft before use 2oo~ Water tou(Jhened&tructure
176
f ~
TRIBOLOGY international August 1976
Woter toughenedond cold wor~ed struclure
being marketed under the trade name of 'FAM Bearings'.
Industrial applications Components produced in this manner have excellent scuffing, wear, and abrasion resistant properties, which the following applications to industrial problems serve to illustrate: Example 1 : Couplings for conveyors built up from metal plates. Sliding speed: 0.05 m/s Load: 1 0 0 - 3 0 0 bars Environment: Highly abrasive Temperature : Ambient
Fig 2O Composite bearings fitted with a liner in spiral form of drawn manganese steel tape
(a) Conventional designs: There are two types involving either surface hardened pins running in bronze bushes fitted with greasing facilities or sealed bearings. In both cases rapid wear occurs, particularly of the bronze bushes. (b) Designs involving use of the HEF preconditioning process: Two variations have been tested, one involving both pins and bushes of nranganese steel (Z 120 M 12) treated by the HEF process, and the other where surface hardened pins run in processed manganese steel bushes. In both cases the need for greasing was eliminated, there was increased reliability, and the life was increased by three or even five times. Example 2: Couplings used on steelworks billet transporters.
Fig 21
HEF treated manganese steel leaf spring shackles for lorries
Fig22
Conveyor coupling showing knurled finish
Sliding speed: 0.1 m/s Load: 5 0 - 2 0 0 bars Environment: Dusty Temperature: Variable from 50-250°C Lubrication: Unreliable (a) Conventional design: As in example 1, there are two types, one using a steel pin and a bronze bush with provision for greasing, and the other consisting of sealed bearings. Rapid wear of the pins and bushes occurs, and there is seizing of the sealed bearings. The average life of either type is 3 - 5 months. (b) Design involving the use of the HEF preconditioning process: This consists of a pin made from manganese steel Z 120 M 12 treated by the HEF process in combination with an FAM bearing. Again there is no need to use grease and the life is three or four times as great.
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Applications A brief list of current applications is given below. It must be borne in mind that the process is still relatively new and different applications are constantly being tested. Automobile: Leaf spring shackles, suspension couplings, lorry skip trunnions Steel Industry: Conveyor parts for rolling mills, coke oven chargingmachine parts, brake couplings Public Works: Mechanical shovel joints, jack couplings, crank pins and bushes Cement Industry: Pin and bush linkages for steel plate conveyors used in cement manufacture Glass Industry: Bearings of silica extractors, charging-machine parts Foundry: Casting machine swivel parts Agricultural Industry: Various moving parts
Fig 23 Manganese steel bush treated by the HEF process, used for disc brakes with an induction hardened carbon steel pin
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TRIBOLOGY international August 1976