Month: December 2018

Interviews

Interview with Zvi of Zknives.com

Larrin

Thanks to the Knife Steel Nerds Patreon supporters for an awesome 2018!

Introduction

Zvi, who goes by Gator97 on many forums, developed the website Zknives.com which contains many articles on various topics including many on knives and sharpening. He also has a steel composition database which is the largest ever made collecting essentially any steel ever used in knives. As far as I know it is the largest database of steel compositions of any type. The database is available on his website and as Android and iOS apps. Because of that very useful contribution to knife steel nerds, I asked Zvi if he would be willing to be interviewed and he graciously agreed. read more

Steel and Knife Properties, Toughness

Why Cold Steel Is Brittle

Larrin

Thanks to Tracy Mickley from becoming a Knife Steel Nerds Patreon supporter!

Effect of Temperature on Strength

Steels become stronger at lower temperatures. This can be measured with the “yield strength” which is the load to permanently deform the steel. This deformation is in the form of a shape change, ie. if you are bending steel it stays bent, when hammering hot steel it dents, when flexing an edge it rolls. This is perhaps easier to visualize for those that have forged steel because hot steel is easier to forge, and is progressively more difficult to forge as it cools down. This increase in strength at lower temperatures continues below room temperature, so steel at cryogenic temperatures is stronger than at room temperature. Here are values for yield stress for 410 stainless steel heat treated to 39 Rc [1]: read more

Cryo, Edge Retention

Cryogenic Processing of Steel Part 3 – Wear Resistance and Edge Retention

Larrin

Thanks to Ian Rogers for becoming a Knife Steel Nerds Patreon supporter!

Intro to Cryo and Wear Resistance

In Cryogenic Processing Part 1 I covered the effects of cryo on retained austenite and hardness. In Cryogenic Processing Part 2 I looked at the studies on cryo and toughness. Wear resistance is the most controversial aspect of cryogenic processing of steel. In particular there are claims that the use of cryogenic processing (liquid nitrogen) leads to an improvement in wear resistance that is not found with subzero processing (dry ice). Sometimes it is claimed that cryo can lead to massive increases in wear resistance [1]: read more

Cryo, Tempering, Toughness

Cryogenic Processing of Steel Part 2 – Toughness and Strength

Larrin

Thanks to Gator, Russell Dodd, and Matt de Clercq for becoming Knife Steel Nerds Patreon supporters!

Introduction

Part 1 of the Cryogenic Processing series covered the transformation of retained austenite to martensite and the increase in hardness that occurs. That is the least controversial aspect of cryogenic processing of steel. The other two primary properties of steel affected by cryo processing are toughness and wear resistance. Both of these properties can be difficult to pin down as they have high variability. Tool steels are known for their relatively poor toughness which means we are often comparing small numbers.

Detour – Tempering

One important interrelation to keep in mind with subzero and cryo studies is the transformation of retained austenite in tempering. With sufficiently high tempering temperatures all/most of the retained austenite is transformed without any cold treatment. This depends on the alloy content, as low-alloy 52100 will have lost its retained austenite with a 500-600°F temper while high alloy steels need over 900°F. You can read more in the article on tempering. With high alloy steels the loss of retained austenite also coincides with “secondary hardening” which is a high temperature tempering treatment that increases hardness [1]:

Above is a tempering chart for Caldie steel (0.7C-5.0Cr-2.3Mo) which shows both hardness vs hardening temperature and also retained austenite. You can see that at low tempering temperatures (<400°C) the retained austenite is basically constant. You can also see that the hardness decreases with higher tempering temperatures up to about 350°C and then it increases to a peak at around 520°C (950°F). Therefore tempering in the secondary hardening region above 400°C can lead to both high hardness and also the elimination of retained austenite.

Subzero or cryo processing prior to tempering also shifts the tempering-hardness curve to lower temperatures when using the secondary hardening range of tempering [2]:

This means that in general, a lower tempering temperature is required to achieve the same hardness level with secondary hardening. Using the same tempering temperature as without a subzero treatment will lead to a greater degree of tempering. More tempering can be good or bad depending on the situation. Excessive tempering can lead to coarsening of tempering carbides which can reduce toughness. However, if the tempering was insufficient without subzero, the use of subzero processing may increase toughness due to shifting the “optimal toughness” range.

Toughness

In an earlier article where we tested the effects of heat treatment on Z-Wear toughness read more

Cryo, Hardness, Heat Treating and Processing

Cryogenic Processing of Steel Part 1 – Maximizing Hardness

Larrin

Thanks to Hiroaki Misono and Robert Venable for becoming Knife Steel Nerds Patreon supporters!

Heat Treating and Austenitizing

During heat treatment of steel, the steel is heated to a high temperature called the “austenitizing” temperature where a phase called austenite is formed. Steel has different phases which refer to different arrangements of iron atoms within the steel. Austenite has a different set of properties from the typical room temperature phase of steel. One example of the different properties of austenite is that it is non-magnetic unlike the room temperature ferrite or martensite.

Room temperature iron/steel – Ferrite – Body Centered Cubic Atom Arrangement

High temperature iron/steel – Austenite – Face Centered Cubic Atom Arrangement

After holding the steel at the high austenitizing temperature, the steel is then rapidly quenched which transforms the steel to a phase called martensite which has high hardness. It gains its high hardness because carbon is trapped in between the atoms which makes the room temperature phase martensite as opposed to the soft ferrite.

Normal soft room temperature ferrite on the left and hard martensite on the right read more