Basic metallurgy you need to know
Category: Metallurgy Principles
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Rockwell Hardness is the Megapixels of Knife Steel Specs
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Hardness and Megapixels
In the early-to-mid 2000’s with digital cameras and somewhat more recently with smartphone cameras we had the battle of megapixels. The number of megapixels is simply the number of pixels that are captured by a digital camera. When we had 0.3 megapixel cameras the pictures were quite blurry and jumping up to 2 or 3 megapixels made a big difference. However, when comparing 5 to 7 megapixels the quality of the image was much more likely to be controlled by the quality of the lens and sensor than simply the number of megapixels. Despite that, megapixels became an easy marketing point because it is a simple number to present to the public. We haven’t seen rockwell hardness climbing for no reason other than marketing, but it is one of the few simple numbers that are used to advertise for a knife. Therefore it is often misunderstood by knife buyers, and yes, even some knife makers. In this article I cover some simple reasons why hardness is not as important as other factors for predicting most steel properties. And then we get into the nitty gritty with why hardness is not always the same as strength and how heat treatment can affect strength independent of hardness.
Review – Kevin Cashen’s Guide to 1080 & 1084
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Misc. updates: I added some toughness numbers that I had previously been unable to track down comparing 440C and 154CM to the 154CM article. I also added a summary of a very interesting new journal article about the effect of grain size on steel toughness to the Grain Refinement article.
How Much More Chromium Does D2 Need to be Stainless?
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Some materials like aluminum form a passive oxide layer that prevents further corrosion. Steel is not one of those materials. Instead, steel forms iron oxide, or rust, that doesn’t protect the underlying iron and flakes off leading to further corrosion. However, when sufficient chromium is added then a chromium oxide passive layer forms which protects the steel from corrosion in a similar way to a metal like aluminum with its own aluminum oxide layer. A simple schematic diagram shows the passive film vs rust [1]:
Is Spider Silk Stronger than Steel?
This article was discussed on the very popular Skeptic’ s Guide to the Universe podcast: https://www.theskepticsguide.org/podcast/sgu/670
Nearly a year later they briefly discussed the article as being very impactful in correcting some common knowledge they had always believed: https://www.theskepticsguide.org/podcast/sgu/714
What Makes Quenched Steel so Hard?
Update: I have started a Patreon page to fund research projects which you can read about here – http://knifesteelnerds.com/how-you-can-help/
To harden steel you heat it up to high temperature to form a phase called austenite, followed by rapid quenching to make a very strong phase called martensite. Hardness is a measure of strength. I covered the process of austenite formation in the following post: Austenitizing Part 1. To summarize that post:
How Does Grain Refinement Lead to Improved Properties?
Update 6/21/2018: A new journal article has been released on the effect of grain size which is very interesting. I have added a brief summary of it at the bottom of this article.
In my posts on austenitizing I described parameters of heat treating to keep grain size as small as possible and therefore improve strength and toughness [1-3]. It is very difficult to improve both strength and toughness at the same time, usually increasing one decreases the other. By what mechanism does grain refinement improve both?
Why Doesn’t Heat Treating Affect Steel “Flex?”
One concept that can be difficult to understand intuitively is that the hardness or strength of steel does not change its stiffness. Meaning, “flexing” steel without permanently bending it is not changed by its hardness. I have heard many people swear up and down that they can tell a difference, so I found a great Youtube video that illustrates this [1]:
Hi larrin,
Congratulations on your website.
I put this question out once on another forum without receiving a reply from metallurgist so, here I go again.
What are the chances of inducing temper embrittlement when flame softening a knife tang , or inadvertently overheating an edge whilst dry grinding?
Any comments welcome
Best Regards
John
Hi John,
It is as likely as heating the blade into the embrittlement range, approximately 500F. Without measuring the temperature during your grinding or flame softening practice it’s hard to know what temperatures you will be seeing. I know there are plenty of knifemakers that state that if grinding without gloves you are unlikely to overheat the edge but that may not be a guarantee particularly for thin edges. To get any real softening in a tang you likely have to temper significantly hotter than the embrittlement range. Also, tempering in the embrittlement range doesn’t necessarily take the blade from tough to brittle, it is simply a range of tempering where there is a small reduction in toughness despite the steel becoming softer, so for a tang I wouldn’t worry much. For an edge the concern would be reaching temperatures higher than the intended temper, and therefore achieving properties different than designed. It would be a fun experiment to try to measure temperatures at the edge while grinding.
Hi Larrin,
I very much appreciate you thoughts on the matter. What led mo the the question is the observation that some blades, presumably ground dry, exhibit edge chipping for some time, but with a few sharpenings, this goes away.
Cheers
John
That certainly sounds like a sign of overheating.
Larrin,
I was a bit too fast with the posting and afterwards did not know how to edit what I wrote, so treat this as part of my reply:
I understand what you said about the softening of the tang, but of course there will be a transitional HAZ that would fall right into the temper embrittkement zone.
Cheers
John