The effects of heat treatment on fracture toughness and fatigue crack growth Rates in 440C and BG42 steels |
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Authors: | Bingzhe Lou B L Averbach |
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Affiliation: | (1) Shaanxi Mechanical Engineering Institute, Xian, People’s Republic of China;(2) Massachusetts Institute of Technology, 02139 Cambridge, MA |
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Abstract: | The fatigue crack growth rates,da/dN, and the fracture toughness, KIc have been measured in two high-carbon martensitic stainless steels, 440C and BG42. Variations in the retained austenite contents
were achieved by using combinations of austenitizing temperatures, refrigeration cycles, and tempering temperatures. In nonrefrigerated
440C tempered at 150 °C, about 10 vol pct retained austenite was transformed to martensite at the fracture surfaces duringK
Ic testing, and this strain-induced transformation contributed significantly to the fracture toughness. The strain-induced transformation
was progressively less as the tempering temperature was raised to 450 °C, and at the secondary hardening peak, 500 °C, strain-induced
transformation was not observed. In nonrefrigerated 440C austenitized at 1065 °C,K
Ic had a peak value of 30 MPa m1/2 on tempering at 150 °C and a minimum of 18 MPa m1/2 on tempering at 500 °C. Refrigerated 440C retained about 5 pct austenite, and did not exhibit strain-induced transformation
at the fracture surfaces for any tempering temperature. TheK
Ic values for corresponding tempering temperatures up to the secondary peak in refrigerated steels were consistently lower than
in nonrefrigerated steels. All of the BG42 specimens were refrigerated and double or quadruple tempered in the secondary hardening
region; theK
Ic values were 16 to 18 MPa m1/2 at the secondary peak. Tempered martensite embrittlement (TME) was observed in both refrigerated and nonrefrigerated 440C,
and it was shown that austenite transformation does not play a role in the TME mechanism in this steel. Fatigue crack propagation
rates in 440C in the power law regime were the same for refrigerated and nonrefrigerated steels and were relatively insensitive
to tempering temperatures up to 500 °C. Above the secondary peak, however, the fatigue crack growth rates exhibited consistently
lower values, and this was a consequence of the tempering of the martensite and the lower hardness. Nonrefrigerated steels
showed slightly higher threshold values, ΔKth, and this was ascribed to the development of compressive residual stresses and increased surface roughening in steels which
exhibit a strain-induced martensitic transformation. |
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