Strengthening in low carbon pearlitic steels

Materials Science and Engineering, 84 ( 1 9 8 6 ) 1 3 1 - 1 3 5

Strengthening

in Low Carbon Pearlitic

131

Steels

B. E. O'DONNELLY* and T. N. BAKER

Department of Metallurgy, University of Strathclyde, Glasgow (U.K.)

(Received February 25, 1986; in revised form April 11, 1986)

ABSTRACT

The microstructure-flow stress relationship has been examined in two simple ferritepearlite steels. As in fully pearlite steels, a Hall-Perch equation was shown to be applicable, when care is taken in obtaining the appropriate mean slip distance in ferrite for each microstructure.

1273 K for 20 min and then air, vermiculite or furnace cooled to r o o m temperature. The cooling rates were determined from axially e m b e d d e d thermocouple probes. Blanks were machined to standard tensile test pieces of gauge length 1.6 cm and pulled to fracture at a cross-head speed of 0.5 mm min -1. Heat treatment data are presented in Table 2.

3. METALLOGRAPHY 1. INTRODUCTION

The strengthening mechanisms in fully pearlitic steels have been the subject of numerous publications and are well understood. Previous studies have shown that, when the interlamellar spacing is considered to be the effective ferrite grain size in a Hall-Petch [ 1, 2] equation, satisfactory correlations are found between microstructural measurements and flow stress data [3, 4]. However, in steels where comparable amounts of ferrite and pearlite are present, some difficulty has been encountered in accurately accounting for the relative contributions of each phase to the flow stress of the composite. In the present study this problem is examined in t w o simple structural steels and the relevance of applying to these steels structureproperty models which were developed previously is considered [5, 6].

Quantitative metallography analysis was c o n d u c t e d by optical and scanning electron microscopy. The volume fractions of ferrite and pearlite were measured using systematic two-dimensional point counting, and the mean ferrite grain size d~ was determined by a standard linear intercept technique [7]. Standard deviations were used to evaluate the 95% confidence limits. As the slip planes in pearlitic ferrite will be effectively random with respect to the cementite lamellae [8], mean random interlamellar spacing measurements were performed on a scanning electron microscope. 40 randomly selected fields were counted to obtain 1000 intercepts. Again, standard deviations were used to obtain the 95% confidence limits. Microstructural measurements are given in Table 3.

4. RESULTS AND DISCUSSION 2. EXPERIMENTAL DETAILS

The chemical analyses of the steels studied are given in Table 1. Samples of each composition were austenitized at 1173, 1223 or *Present address: Olin Corporation, Metals Research Laboratories, N e w Haven, C T 06511, U.S.A. 0025-5416/86/$3.50

As expected, the yield strength of each steel increased as the ferrite grain size decreased. A Hall-Petch plot of these data is shown in Fig. 1. Although a reasonable correlation is obtained (coefficient, 0.78), it will be n o t e d that, even when solid solution elements are considered, the 0.31 wt.% C steel has in general a greater strength for approxiQ Elsevier Sequoia/Printed in The Netherlands

132 TABLE 1 C o m p o s i t i o n s o f l o w c a r b o n steels

Steel

A B

A m o u n t (wt.%) o f the following elements C

Si

Mn

P

S

Cr

Mo

Cu

Sn

0.19 0.31

0.46 0.36

0.89 0.86

0.017 0.019

0.015 0.016

0.03 0.03

0.02 0.004

0.06 0.05

0.006 0.007

TABLE 2 H e a t t r e a t m e n t a n d m e c h a n i c a l p r o p e r t y d a t a f o r 0.19 wt.% C steel (A) a n d 0.31 wt.% C steel (B)

Specimen

Austenitizing temperature

Cooling

Yield stress

(K)

Method

Rate (K s-1)

A1 A2 A3 A4 A5 A6

1173 1173 1223 1223 1273 1273

Vermiculite Air Vermiculite Furnace Vermiculite Air

~ 0.2 ~ 3.3 -~ 0.1 ---

330 337 318 288 305 317

B1 B2 B3 B4

1173 1223 1223 1273

Vermiculite Vermiculite Furnace Furnace

~ ~ ~ ~

356 332 324 328

(N m m -2)

0.3 0.3 0.1 0.3

TABLE 3 Measured and calculated microstructural data

Specimen

Measured data Ferrite volume fraction Va

Calculated data Ferrite grain size d~ (~tm)

Mean random p_earlite spacing Sr (nm)

Mean random cementite thickness [r (nm)

Mean slip distance ~a in ferrite (~tm)

A1 A2 A3 A4 A5 A6

0.751 0.743 0.760 0.771 0.784 0.765

17.2 15.5 23.0 27.6 26.2 19.1

577 475 497 597 542 486

66.0 52.7 59.0 74.3 71.5 58.9

13.0 11.6 17.6 21.4 20.6 14.7

B1 B2 B3 B4

0,624 0.615 0.630 0.599

15.1 25.0 24.1 20.6

582 622 617 578

72.0 75.1 77.5 67.0

9.6 15.6 15.4 12.5

+10%

-+4%

95% confidence limits

133

z

o

8 FERRITE GRAIN SIZE dQ ,2 {ram 12)

Fig. 1. L o w e r yield stress as a f u n c t i o n o f t h e m e a n ferrite grain size in t h e 0.19 wt.% C steel ( o ) a n d 0.31 wt.% C steel (o).

mately the same mean ferrite grain size than the lower carbon steel has. This is obviously a result of the increase in the volume fraction of pearlite with increase in carbon content and such effects have already been n o t e d by other investigators [9, 10]. To account for variations in pearlite volume fraction, a number of researchers have employed multiple linear regression analysis on their data [3, 9, 11]. However, the w o r k of O'Donnelly et al. [5] has demonstrated that the flow stress ac of ferrite-pearlite steels may be described by a Hall-Petch equation of the form O c = O 0 + K X a -1/2

(1)

where o0 is the frictional stress for pure ferrite, K is a constant and X~ is the mean ferrite slip distance in the composite. These researchers [5] showed that ~ can be obtained from a modified law of mixtures, i.e. X~ =

volume ~ I mean slip fraction ~ X ~ distance of pearlite) [ in pearlite

?

.


o

3oo

I

,

I ~.

_

i

I q

,

I !r

Fig. 3. L o w e r yield stress as a f u n c t i o n of t h e m e a n slip d i s t a n c e in ferrite f o r 0 . 1 9 wt.% C steel (o) and 0.31 wt.% C steel (o).

prior austenite grain size. tr, the mean random cementite lamella width, can be calculated from [5]

Vp (3)

For microstructures where proeutectoid ferrite is distributed as a n e t w o r k around prior austenite grain boundaries,

=

I

0.15 X weight percentage of carbon X S~

~volume } {meanslip} + ~fraction ~ X distance ( of ferrite) in ferrite

X~ = V , ( S ~ - t,) + V~(V~L~)

Fig. 2. F e r r i t e - p e a r l i t e m i c r o s t r u c t u r e in t h e 0.31 wt.% C steel c o o l e d in v e r m i c u l i t e f r o m 1 2 2 3 K.

(2)

+

where Vp and V~ are the volume fractions of pearlite and ferrite respectively, S~ is the mean random interlamellar spacing and/,~ is the

In the microstructures of the present study, as shown in Fig. 2, the ferrite was not confined to the prior austenite grain boundaries. Therefore, the appropriate form of eqn. (2) is [12] = vp(

r-

+

(4)

To apply the above analysis to the steels in the present study, measured microstructural parameters were used to calculate t-r and Xa from eqns. (3) and (4). These values are given in Table 3. Figure 3 shows a c as a function of Xa (from eqn. (1)). Evidently employing Xa

134 900 800

800

7~C,

700

D 0

4TE

500

%%

630

65% PEARLJTE

500 500

qO0 o ~ o

4)0

I

300

fo oo

200

300 100 233 ZO

20

30

qO

50

~

70

80

2'0

~-t/2 (ram-l/2)

40

60

80

100

dc i/2 (ram '/~I

Fig. 4. Yield stress as a function of the mean slip distance in ferrite for a range of pearlitic steels: o, 0.42 wt.% C steel; m, 0.59 wt.% C steel; A, 0.75 wt.% C s t e e l ; • , 0.82 wt.% C steel;o, 0.19 wt.% C steel; o, 0.31 wt.% C steel.

Fig. 5. Yield stress as a function of the composite grain size for a range of pearlitic steels: o, 0.19 wt.% C steel; o, 0.31 wt.% C steel; [], 95-100 vol.% pearlite; m, 90-95 vol.% pearlite; ~, 80-90 vol.% pearlite; • , 65-80 vol.% pearlite.

instead of simply d~ gives an improved correlation with yield stress values (i.e. a correlation coefficient of 0.93 compared with 0.78 for Fig. 1). Indeed, when the results o f the present work are combined with data obtained in an earlier study on high carbon pearlitic steels [5] (Fig. 4), it becomes clear that eqns. (1)-(4) are applicable over a wide range of pearlite volume fractions. Unfortunately, the addition of low carbon steel data does n o t improve the frictional stress predicted b y Fig. 4, which is higher than normal. The reason for this is n o t entirely clear but, as discussed in a previous report [5], m a y be because the single linear relationship shown in Fig. 4 does in fact consist of a family of curves each with a slightly different slope according to the volume fraction of pearlite present in the alloy. Theoretical justification for this argument can be f o u n d in the work of R e u b e n and Baker [6]. These researchers developed an equation for the flow stress of ferrite-pearlite steels of the form

steels. They showed that when care is taken in accurately determining the mean slip distance in the pearlite, i.e. through S r - t-r, the behaviour of ferrite-pearlite steels is consistent with that of 100 vol.% ferrite. The results of their work, together with the data obtained in the present study, are compiled in Fig. 5. (For the solid solution element contents of the steels in the present study the coefficient ks and kp were taken to be 17.5 and 7.9 respectively [3, 5, 12]. An approximate value of 0.2 for Vp was used in eqn. (5).) Again, satisfactory agreement is found between microstructural measurements and flow stress data. Although it would be difficult on a purely predictive basis to reject eqns. (2) and (4) in favour of eqns. (5) and (6), theoretically the latter equations are more tenable. Furthermore, as found in the preceding studies [5, 6], when solid-solution-strengthening element concentrations are considered, the frictional stress obtained for all pearlite-containing steel is very close to that associated with fully ferritic microstructures. A final point to note from the above results is the need to include the mean slip distance both in ferrite and in pearlite in determining the effective grain size in a Hall-Perch analysis. Even when the volume fraction of either constituent is less than 0.1, such small amounts still have an appreciable effect on the value of the mean slip distance in the microstructure and this is reflected in the flow stress of the steel.

Gc -- O0 4- ( k s -}- (kp - - k a ) Y p } d c --112

(5)

where ks and kp are the Hall-Petch coefficients corresponding to 100 vol.% ferrite and 100 vol.% pearlite respectively. The composite grain size in this case is given by [5, 6] dc 1/2 -- Vp(~r -- Er)1/2 + V~(d~ or V~LT) 1/2

(6)

O'Donnelly et al. [5] and Reuben and Baker [6] have applied eqns. (5) and (6) to their microstructural and flow stress measurements in medium to high carbon pearlitic

135 5. CONCLUSIONS T h e c o n c l u s i o n s o f t h e p r e s e n t s t u d y are c o n s i s t e n t ~vith t h o s e d r a w n in a p r e v i o u s report and show that a Hall-Petch equation is a p p l i c a b l e t o a wide r a n g e o f pearlitic steels. W h e n care is t a k e n in d e t e r m i n i n g t h e a p p r o p r i a t e m e a n slip d i s t a n c e f o r e a c h m i c r o s t r u c t u r e , a s a t i s f a c t o r y c o r r e l a t i o n is f o u n d w i t h t h e f l o w stress data.

ACKNOWLEDGMENTS T h e a u t h o r s are g r a t e f u l t o Dr. R. L. R e u b e n a n d Dr. R. R. P r e s t o n f o r v a l u a b l e discussions a n d t o P r o f e s s o r H. B. Bell, Department of Metallurgy, University of Strathc l y d e , f o r t h e use o f l a b o r a t o r y facilities. B. E. O ' D o n n e U y g r a t e f u l l y a c k n o w l e d g e s t h e f i n a n c i a l assistance o f t h e Science a n d E n g i n e e r i n g R e s e a r c h C o u n c i l a n d British Steel C o r p o r a t i o n .

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