The effect of phosphorus content on grain boundary cementite formation in AISI 52100 steel
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TEIICHI ANDO AND GEORGE KRAUSS Intergranular fracture surfaces of high phosphorus (0.023 wt pct P) and low phosphorus (0.009 wt pct P) AISI 52100 steels were investigated by Auger Electron Spectroscopy (AES). Cementite, identified by composition and Auger peak shape, was found to form on austenite boundaries in specimens oil quenched from 960 ~ to room temperature as well as in specimens quenched from 960 ~ and isothermally held at temperatures between A cmand A r Phosphorus segregates to austenite boundaries during austenitizing and accelerates cementite formation on the austenite boundaries. Concentration profiles obtained by AES during ion sputtering showed that phosphorus may be incorporated in the first-formed cementite and concentrates at cementite/matrix interfaces in later stages of cementite growth. The amount of interphase P segregation in the later stages is proportional to bulk alloy P concentration in accord with McLean's theory of grain boundary segregation in dilute alloys and appears to approach equilibrium at high reaction temperatures (785 ~ At lower reaction temperatures (740 ~ the interphase segregation is lower than expected, a result that may be attributed to reduced diffusivity of P at the lower reaction temperature.
F R A C T U R E of high carbon steel often occurs along prior austenite boundaries) -5 The intergranular fracture is considered to be due to a combination of segregation of impurtiy elements such as P and subsequent carbide formation along austenite grain boundaries? ,5 Cooling rate experiments sugges: that P segregation during austenitization may promote or accelerate the formation Of a very thin film of grain boundary cementite during quenching. The cementite films have as yet not been directly revealed, but AES observations provide indirect evidence for their presence: Much of the AES work which characterizes grain boundary impurity segregation has been performed on tempered-martensite-embrittled6 and temper-embrittled7 medium carbon steels. Few AES investigations of impurity segregation in high carbon steels have been performed. Krauss 3 has shown C and P concentration at prior austenite grain boundaries in association with intergranular fracture in a carburized steel containing 0.01 wt pct P. Since the steel was tempered at 145 ~ a range normally considered to be free from the intergranular form of TME in medium carbon steels, 6,s it was not clear if the carbon segregation associated with the intergranular fracture developed on tempering or prior to tempering during another stage of the heat treatment process. Another investigation9 of carburized steel showed no apparent effect of impurities on intergranular fracture, and although not commented on by the authors, did show C concentration higher than bulk on the fracture surfaces. Suto et a l to showed that in a series of high C steels
TEIICHI ANDO, Research Assistant, and GEORGE KRAUSS, AMAX Foundation Professor, are with the Department of Metallurgical Engineering, Colorado School of Mines, Golden, CO 80401. Manuscript su
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