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  1. This thread is for a discussion on the causes and effects of temper embrittlement. The topic comes up every now and then, and really deserves a thread to be able to point people to when needed. I will do my best to keep things as simple as possible, while at the same time covering the important parts. Besides a number of years as a practicing metallurgist in steel foundries I am also using my 3 most common reference books for this type of topic that I keep within reach on my desk: The ASM Heat Treater's Guide (2nd ed, 1995), Structure and Properties of Engineering Alloys (2nd ed, William F. Smith, 1993), and Phase Transformations in Metals and Alloys (2nd ed, 1992). For my own sanity I will not be trying to be overly diligent to indicate which source I am referencing in the details. It should be pointed out right off the bat that there is still some things that are not known about the mechanisms behind it, but that it actually exists and has negative effects is not under any debate amongst metallurgists. All temper embrittlement is based on quenched and tempered steel. The embrittlement affects martensite in steel, no need to worry about normalized steels. Blue Brittleness I will admit that I am quite terrible about continuing to use this term as a catch-all for temper embrittlement. I will try to do better about that. Blue brittleness is noted by ASTM to form between about 400-700 F, but generally is not a problem with properly deoxidized steels. Therefore true "blue brittle" problems are not likely to be encountered with modern steels produced in good mills. Generally when this is referred to (at least by me, and I am sure many others) what is really meant is a general temper embrittlement that will be discussed below. The term blue-brittle comes from the fact that steel that has fractured due to this phenomenon tends to have a blue hue to the fracture surface due to oxide growth at that temperature. Other temper embrittlement mechanisms don't show this. One Step Temper Embrittlement Generally formed between 440 and 660F in short temper times. The embrittlement has experimentally been determined to be related to impurities. As summarized by Smith: The occurrence of the anomalous impact-energy trough coincides with the beginning of cementite precipitation. Since the one-step embrittlement causes an intergranular mode of fracture along prior austenitic grain boundaries, it is believed that the segregation of P, N, and possibly S to the austenitic grain boundaries is essential for this type of embrittlement. Alloying elements such as manganese may have an indirect effect by promoting the segregation of the embrittling elements to the grain boundaries. The presence of the undissolved carbides at the prior austenitic grain boundaries is thought to accentuate the impurity-induced intergranular fracture, the carbides acting as slip barriers. Now, I know that that is a bit of a tough "summary". Basically it indicates that one should really avoid the 440-660 F range when one has too much (an actual number is not given) of P, N, and S, and the "too much" threshold is probably lowered as Mn goes up. Also, if one has carbides that have not been properly dissolved prior to hardening then this one-step mode of embrittlement is more likely to be a problem. Two Step Temper Embrittlement Smith indicates that it occurs in about the 700-1000 F range. Every metallurgist I have spoken to actually puts the upper limit in the 800-900 F range, with 900 F being a common temper range. It is best to spend as little time in this range as possible when tempering; on heating and cooling (more important than the heating speed), and certainly don't hold in this range. This, too, is an embrittlement mechanism related to impurities and their segregation to the grain boundaries. As summarized by Smith: The ductile-brittle transition temperature is directly dependent on the grain boundary concentration of the impurities. This effect [was tested] a nickel-chromium steel doped with Sb, Sn, and P. The relative effect of these impurities was found to be Sn > Sb > P. Alloying elements sometimes cosegregate to the grain boundaries with the impurities. For example, nickel cosegregates with antimony (Sb). The segregation of impurities to the grain boundaries appears to be an equilibrium phenomenon. The equilibrium grain boundary concentration of impurities increases with decreasing aging temperature. Time also is important at lower temperatures. For example, Increased aging time increases the concentration of Sb in a 3.5% Ni - 1.7% Cr - 0.008%C - 0.06% Sb steel. Again, that is a bit dense. The take-away is that mechanism is again impurity driven and the lower end of the temperature range is more damaging than the upper end of the range. Also mentioned by Smith is that Ni, Cr, and Mn increase the effects of two step embrittlement. Mo inhibits the embrittlement by precipitating out as phosphides (thereby tying up the P). The two-step embrittlement is the most likely form we have to worry about with modern steels. I can speak anecdotally to the effects of temper embrittlement, having either performed or reviewed results for hundreds of tests where this was a potential issue. When temper embrittlement occurs, everything about ductility (elongation, reduction of area, bending) and impact toughness suffers. Tangentially related to temper embrittlement is hydrogen embrittlement ("rock candy" failure). This is when (monatomic) hydrogen causes a problem in steel. It is often removed at 500-600 F, as that is the fastest temperature at which it will diffuse out of the steel. This still takes days in many cases. Manufacturers put in ovens just for heating these parts. I mention it here because they all have to soak these parts at that temperature for a long time before they harden the parts because they know that they can't do it after the fact due to temper embrittlement. I have been in a lot of professional heat treating facilities (most often foundries with their own HT operation). While I have seen a good number of questionable (and down-right horrible) practices, I have yet to see a professional operation that did not avoid this temperature range post hardening. Please let me know if anyone has any questions. I can certainly dig a little more if anyone would like to know more. I have access to several more resources, including potentially newer studies and tests, but I stuck with the 3 books on my desk today and my own experiences for now.
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