Improvement of mechanical properties of type 403 stainless steel by pulse-induction hardening

Scripta METALLURGICA et MATERIALIA

Vol.

25, pp. 2445-2450, 1991 Printed in the U.S.A..

Pergamon Press plc All rights reserved

IMPROYR~.NT OF MECHANICALPROPERTIES OF TYPE 403 STAINLESS STEEL BY PULSE-INDUCTIONHARDENING

L. A l b e r t i n , P. K. Liaw, and M. G. Burke Westinghouse Science and Technology Center P i t t s b u r g h , PA 15235 and R. L. F r o h l i c h WestinEhouse Marine D i v i s i o n Sunnyvale, CA 94088

(Received May 6, 1991) (Revised August 22, 1991) Introduction Pulse hardeninE i s a high power d e n s i t y i n d u c t i o n technique which heats a l o c a l surface area of an i r o n ~ l l o y by s h o r t , high frequency i n d u c t i o n p u l s e s . To explore the e f f e c t of pulse induction hardening on s t e e l s , a proEr~a was i n i t i a t e d t o h e a t - t r e a t notched t e n s i l e and fatiEue specimens made of Type 403 stainless steel by the technique. Specimens were pulseinduction hardened and then tested in tension and fatigue, respectively. In addition, the microstructure and hardness of the s t e e l were evaluated before and a f t e r pulse hardening. Residual s t r e s s e s i n the p u l s e - l n d u c t i o n hardened sone were also measured. The purpose of t h i s paper i s to p r e s e n t the t e n s i l e and f a t i E u e r e s u l t s and t o show the m e t a l l u r g i c a l changes produced i n the s t e e l by the unique h a r d s n i nE process. Experimental Procedure The cheListry of Type 403 s t a i n l e s s s t e e l evaluated i n t h i s study i s shown i n Table I . The mechanical p r o p e r t i e s are glven i n Table I I . The s t e e l was a u s t e n i t l s e d at gSO°C, quenched i n o i l and tempered a t 663"C. The m i c r o s t r u c t u r e of the s t e e l before pulse hardening i s shown i n FiEure 1, which p r e s e n t s a tempered m a r t e n s i t e s t r u c t u r e .

TABLE I Chemical Composition of Type 403 S t a i n l e s s S t e e l Wt.~ Element

c S P Si

o.12 0.018 0.019 0.12

Mo

O. I0

Ni Mn Cr Fe

0.54 0.48

12.34 Bal.

TABLE II Mechanical Properties of T~pe 403 Stainless Steel Used i n the P u l s e - I n d u c t i o n Hardenin~ Experiment O. 2~ Offset Ultiaate BlonEation Reduction Yield S t r e n ~ h Stren~h ~ of Area, MPa MPa

• R

~f

23.2

2445 0056-9748/91 $5.00 + .00 Copyright (c) 1991 Pergamon Press plc

86.0

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The p u l s e - i n d u c t l o n hardening experiments were c a r r i e d out with notched SF-4 type specimens having a s t r e s s concentration f a c t o r , K~ = 2, and a c o n f i g u r a t i o n shown i n Figure 2. The hardening equipment used was a high-frequency, p u l s e - i n d u c t i o n u n i t from Impulsphysik (Impulsa H-II) capable of producing 40 kW of effective peak power at a fixed oscillation rate of 27.12 MHz with preselected pulse durations of 0.5 to 99 milliseconds. The unit develops very high energy densities by pulsing capacitive stored energy through switching thyrations to a megs3ertz triode oscillator and finally to an output inductor. The pulse hardening setup and parameters used are shown in Figure 3. Material characterisatlon studies before and after pulse hardening were carried out by optical metallography and scanning and transmission electron microscopy. Notch strength evaluations were done hy a conventional tensile test. Residual stresses in the pulseinduction hardened sons were measured by X-ray diffraction techniques. Fatigue strength evaluations were conducted in laboratory air at a frequency of 60 Hz and a load ratio (R = F.in/P~, where P~n and P.= are the applied minimum and maximum loads, respectively) of 0.2. Results and Discussion The microstructure of the l o c a l l y p u l s e - i n d u c t i o n hardened, notched zone i s shown i n Figure 4. The mlcrostructure of the s t e e l was g r e a t l y r e f i n e d i n comparison with Figure 1. Further examination of the m i c r o s t r u c t u r e of Type 403 s t a i n l e s s s t e e l before and a f t e r pulse hardening with the scanning asd t r a n s m i s s i o n e l e c t r o n microscope r e v e a l e d the s t r u c t u r e s seen i n Figures 5 through 8. Before pulse hardening, the SE~ micrograph (Figure 5) showed a mlcrostructure of tempered m a r t e n s i t e i n which carbides decorated the m a r t e n s i t e l a t h boundaries. After pulse hardening (Figure 6), the m i c r o s t r u c t u r e c o n s i s t e d of m a r t e n s i t e and r e t a i n e d a u s t e n i t e . A t r a n s m i s s i o n e l e c t r o n micrograph showed a d i s l o c a t e d l a t h - t y p e martensite before pulse hardening (Figure 7), and a slightly tempered martenslte with a much higher dislocation density and some retained austenite after pulse hardening (Figure 8). X-ray diffraction measurements of the pulse-lnductlon hardened zone showed a residual compressive stress of approximately -275 ~Pa at the surface. The Knoop hardness of the notched zone in Type 403 stainless steel was raised from approximately KHN 270 to KHN 530, as shown in Figure 9. The improvement in hardness of the notched zone was related to the refined grain size and a higher dislocation density following pulse hardening. Moreover, the increase in hardness of the notched zone had a significant effect on the notched tensile strength of the material. The notched ultimate strength increased from 1,168 KPa to 1,358 MPa, as shown i n Table I I I . S i m i l a r l y , the f r a c t u r e s t r e n g t h increased from 955 MPa t o 1,233 MPa. The improvement i n the notched s t r e n g t h of Type 403 s t a i n l e s s s t e e l was also e v i d e n t i n a s i g n i f i c a n t improvement of the s t e e l ' s f a t i g u e p r o p e r t i e s a f t e r pulse hardening (Figure 10). TABLE III Notched T e n s i l e Strength. (K+. = 2), of Not Pulse Hardened and Pulse Hardened Type 403 Stainless Steel Ultimate Fracture Condition Notched Strength Strength KPa KPa Not Pulse Hardened

1,168

955

Pulse Hardened

I, 358

I, 233

The f a t i g u e endurance l i m i t i n c r e a s e from p u l s e - i n d u c t l o n hardening i s estimated to be on the order of SO~. The improvement i n the notch t e n s i l e and f a t i g u e s t r e n g t h of Type 403 stainless steel after pulse-induction hardening is attributed to the refined grain size, improved hardness, the presence of compressive r e s i d u a l s t r e s s e s a t the surface, b e n e f i c i a l phase changes, ~nd a higher d i s l o c a t i o n d e n s i t y . A s i m i l a r t r e n d can be found f o r other steels (1,2).

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The e f f e c t of p u l s e - i n d u c t l o n hardening on the m e t a l l u r E i c a l , t e n s i l e , and f a t i g u e properties of Type 403 stainless steels can be summarized as follows: The process refined the grain size, increased the local hardness, improved the notch tensile strength, introduced compressive residual stresses at the surface, and raised the notched fatigue strength. Acknowledgments We l i k e to thank J. J. Haugh and P. M. Yuzawich f o r metallngraphy and hardness determination, and C. L. Donahue and C. E. Rudd f o r the t e n s i l e and f a t i g u e t e s t s . Impulsphysik G.m.b.H. c a r r i e d out the p u l s e - i n d u c t i o n hardeninE p r o c e s s . I.

2.

References Lu Lianbo, 'Research Report--Pulse Hardening", Heat Treating, p. 27, June 1988. L. Albertin, P. K. Liaw and R. L. Frohlich, "Properties of Pulse Hardened Type 403 and 17-4PH Stainless Steel", Morris E. Pine Symposium, P. K. Liaw, J. R. Weertm~n, H. L. Marcus and J. S. Santner, eds., p. 113, TMS-~TMR., Warrendale, PA, 19@I.

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T .4 IlL

4.4 88.9

Unnotched

Notched, K t = 2

Dimensions in m m

Fig. 1.

Microstructure of Type 403 s t a i n l e s s s t e e l before pulse hLrdeninK,

Fi E. 2.

Specinens used for t e n s i l e and fstiKue t e s t s .

Notch

In(kWt~ RolStlo~: ego

A. Rotation Set-Up

t.Smm ~ - - /~ ( ~

|--I ~

poldtloft Tolerance: O.:t:l mm v~rtl~.~I; o.ga m,n ± o.oo7 mm . % ) Sldwwelt

t\

sa ioductor-to-Work Piece Toiel'sJ',C:Je P~aO Type. Single Sklgle Single

Fig. 3.

Capacitor Voltage, KV 5.0 S.O S.0

Rotation Speed, rpm S0O gOO 600

Pulse Time, me g60 $80 600

CeN Depth, mm 0.38 O.S1 0.64

Pulse hardening set-up and para-eters.

Fig. 4.

Pulse hardened notched sons in Type 403 e t n i n l e s e s t e e l .

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F i l . 5.

Safnicrosraph of Type 403 o t s i n l e u s t e e l before pulse hardening,

Fi I. 6.

S]nfnicroKrsph of Type 403 Brainless s t e e l d t e r pulse hardening.

Fig. 7.

TEJdmicroKraph of Type 403 s t a i n l e s s s t e e l before pulse hardening,

Fig. 8.

T ~ mlcrograph of Type 403 s t a i n l e s s s t e e l a f t e r pulse hardening.

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Knoop Hsrdness (KEN)

(soo s)

~N

~N

Center,

Notch

])own

Surf see

537

532 528 537 520

407

246 261

Lg ~5

Fig. g.

511 301

288

288 276 29O

Rarduess of pulse hardened zone in Type 403 s t a i n l e s s s t e e l . Curve 7 ~ , "."T

7~ -403StainlessStaels TO0

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