Hydrogen induced cracking in a low alloy steel
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I.
INTRODUCTION
CONVENTIONAL
STRESS corrosion cracking (SCC) is particularly severe for high strength steels, which are extremely susceptible even under mild environments. Although efforts to improve the general properties of these steels are not lacking, the effects of such efforts upon the SCC characteristics are often not studied. Especially in the case of 300M steel, little work has been published ~-6 and none examined the effects of modified high temperature heat treatments upon the crack growth behavior. Many factors affect the SCC behavior of steels and these include the alloy strength level, applied stress intensity, microstructure (including the type, size, distribution and shape of carbides, dislocation density, retained austenite, prior austenite grain size, and segregation of both impurity and alloy elements), steel composition, environment, and temperature. For alloy 300M little work has been carried out to analyze the effects of these factors. Even when data are available, there are oftentimes conflicting results describing the influence of these parameters upon the SCC behavior. For instance, some investigators reported an increase in threshold stress intensity (K~c) with an increase in prior austenite grain size, 4'7 while others reported no grain size effect. ~.s In this investigation, the influence of such factors upon the SCC behavior was examined.
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E X P E R I M E N T A L PROCEDURE
The material used was commercially available 300M steel, with the chemical composition given in Table I. Figure 1 is a heat treatment schematic. All austenitization treatments were carried out in an argon atmosphere tube furnace with a flat zone accuracy of +--0.5 ~ Tempering treatments were carried out in a salt bath furnace. Bolt loaded double cantilever beam (DCB) specimens (2.5 mm sidegroove) were used for evaluating K~.... After heat treating, a 0.2 mm crack starter notch was cut, and Table I.
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STEP
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Time ( r a i n s )
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these specimens were then loaded while monitoring the deflection accurately with a cathetometer until a preselected deflection value was achieved. These specimens were then immersed in a 3.5 pet NaC1 solution and left under load for 100 hours, after which they were broken open to measure the hydrogen assisted crack extension. From the arm deflection (v) and the crack length (a) Kisccwas evaluated from the relationship:
vEh[3h(a + 0.6h) 2 + h3] ~/2 4(a + 0.6h) 3 + h2a
where E is the modulus of elasticity and h, the half height of the specimen. Compact tension (CT) specimens conforming to ASTM E 399-78a specifications were used to study the crack growth rate as a function of applied stress intensity. After heat treating, a 0.2 mm slot was cut prior to fatigue precracking in an Instron test system at 10 Hz, with a starting mean load of 682 Kgs and an alternating load of 341 Kgs and decreasing these loads while maintaining the R ratio constant, to minimize any residual stress effect. The final mean and alternating loads were 114 Kgs and 57 Kgs, respecti
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