I have been a member of the ABS for 3 years and I would first like to thank everyone that contributes to the forum topics. They are a tremendous help for accurate information and knowledge.
My question is: I am making a small machete-like blade (about 12 inches total length and approx. 1/8 inch thick) using 8670 steel. Lower hardness on this blade is not an issue and I am more concerned with maxing out the toughness. *I know traditional machetes are in the high 40’s to low 50’s hardness range.
I was looking at the tempering data on 8670 for 53-54 hardness and comparing them with temp. graphs on the effects of Tempered Martensite Embrittlement. I have also been reading in Verhoeven’s book about Tempered Martensite Embrittlement (TME). It looks like in the 250 C / 480 F range (for tempering to 53-54 HRC) I am OK and avoiding any dip in impact toughness from TME. There is another toughness peak when tempering around 450 C / 840 F however, that would put the hardness of the blade in the low 40’s range.
I have also read that TME only occurs when longer tempering times are used (longer than 1 hour cycles) so it may not be as big a factor as I am thinking it is! I also read that TME is not an issue with simple carbon steel (so maybe I should have used 1070 instead)
Do you see an issue with my target hardness range and avoiding any effects TME or am I way off base in my interpretation?
Thanks
Randy Powell
Texas
TME shouldn't be too much of a problem with these alloys. TE (tempering embrittlement) is an issue found in much richer alloys that suffer from precipitation type hardening when heated to the higher tempering range of 650°F and above, it is dealt with by rapid cooling to avoid precipitation of unwanted phases on slow cooling. Could you point me to the literature that mentioned TME issues with longer tempering times? Krausse only briefly touches on it.
"One test is worth 1000 'expert' opinions" Riehle Testing Machines Co.
TME shouldn't be too much of a problem with these alloys. TE (tempering embrittlement) is an issue found in much richer alloys that suffer from precipitation type hardening when heated to the higher tempering range of 650°F and above, it is dealt with by rapid cooling to avoid precipitation of unwanted phases on slow cooling. Could you point me to the literature that mentioned TME issues with longer tempering times? Krausse only briefly touches on it.
Thank you Mr. Cashen for the information.
I feel more comfortable now with the 53-54 HRC range on the 8670 steel. I know the lower hardness I am aiming for is a bit unorthodox, so I wanted to run it by the forum experts for any input and advice.
Below are citations for a couple of articles I had read that discussed information on tempering time and TME issues. Most of this was over my head…
Lescanoa DE and SP Silvettib. 2012. Study of Microestructure and Tempered Martensite Embrittlement in Aisi 15b41 Steel. Procedia Materials Science Volume 1, pp 134-140.
“The phenomenon of temper embrittlement of a martensite phase in steels is linked to effects of composition and microstructure. In particular, when tempering during one or two hours this phenomenon is called Tempered Martensite Embrittlement (TME) and is characterized by a drop in the impact toughness after tempering in a range of temperatures from 250°C to 450°C.â€
Euser VK, Williamson DL, Clarke KD et al. 2019. Effects of Short-Time Tempering on Impact Toughness, Strength, and Phase Evolution of 4340 Steel Within the Tempered Martensite Embrittlement Regime. Metallurgical and Materials Transactions A 50, 3654–3662.
“Toughness improves with short-time tempering compared to conventional tempering within the same strength and hardness regime. In addition, TME is reduced with decreasing tempering time. The increase in overall toughness and decrease in TME severity may be connected to the reduction in retained austenite decomposition to interlath cementite that is associated with short-time tempering.â€
Thanks again for your help!
