Temper Embrittlement Diagram of NiCr Steel Doped with Phosphorus
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large number of recent studies on temper embrittlement of low alloy steels, it has been made clear that the decohesion of grain boundaries is caused by intergranular segregation of metalloid elements such as P or Sn. It is also now widely recognized that the degree of temper brittleness depends mainly on three metallurgical factors, i.e., prior austenitic grain size, hardness, and intergranular concentration of embrittling metalloid impurities. A major concern in engineering research is to use this knowledge to predict and control the temper brittleness of steels containing small amounts of impurities, since the concentrations of some metalloid elements cannot be decreased to completely harmless levels for both technical and economic reasons. Recently it has been reported that Charpy impact tests on NiCr steels doped with P, Sn, and Si and heat-treated to vary the three metallurgical factors systematically made it possible to derive a temper embrittlement equation (TEE) which enables us to calculate with reasonable accuracy the transition temperatures for various combinations of the three variables. J Once the embrittlement equation is derived, a temper embrittlement diagram 2 (TED) can be easily constructed. This paper supplements the previous report on the TEE,~ and it shows the construction of two types of TED for a NiCr steel doped with P; it also shows the usefulness of the TED in predicting and controlling the temper brittleness of steels. It has been made clear that P is by far the most common embrittling element among various harmful impurities. The general form of the TEE derived for the NiCr steels doped with P, Sn, and Si is 0TT Tr(P,, G, H) = Yr(O, G o, H o) + P, OPI oqTT
+ (H - H ~
OH
O2TT
+ P,(G - G~ ~
OPi OG
02TT + Pi(H - H
9 (H - n ~
~
OPi OH 03TT
OPiOG OH
+ Ps(G - G ~)
[11
where TF is the Charpy V-notch transition temperature, Pi the Auger peak height ratio of segregated impurity i at grain boundaries with respect to the Auger peak height of Fe at 703 eV, G the grain size in ASTM number, H the hardness
T. OGURA is Associate Professor, The Research Institute for Iron, Steel and Other Metals, Tohoku University, Sendai 980, Japan. Manuscript submitted April 14, 1982.
METALLURGICAL TRANSACTIONS A
on the Rockwell C scale, G ~ the smallest mean grain size (ASTM number) of the steels examined, and H ~ the lowest hardness of the steels examined. The values of the first term, T/" (0, G ~ H~ and all derivatives in the above equation have been given for P-, Sn-, or Si-doped steels in the preceding paper.~ If we require TT as a function of the atomic fraction of an impurity element i at grain boundaries, Xi, all the derivatives with respect to Pi must be transformed to derivatives with respect to Xi by using the relationship, O/OX~ = (O/OP~) d P i / d X ~ . * The c o n v e r s i o n factor, *This will work if the Pi values were obtained from fracture surfaces that are essentially 100 pct intergranular fracture; if not, then the pct intergranular fracture must be measured separately and
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