Thread: Resistant chipping

Originally Posted by Neil Miller

I had a discussion with a tool, knife and razor maker some time ago about the use of 'pattern welded' steels, especially with regards to the cutting edge....

It is a useful discussion. There are specific conditions that could produce one layer that chips more than another. There are historical precedents for the development and use of patterned steels and for using a mono steel at the edge.

I can't speak for any one individual maker's choices of steels except my own. I'd like to believe that we have all had the same basic education about steels and mixtures in pattern development. I use 1095 and 15N20 (essentially 1075 with 2% nickel). The nickel resists etching and is what creates the contrast between layer "colors." Given carbon migration during welding the billet will end up being at or near 0.85% carbon and that is considered the eutectic sweet spot for carbon steels, neither too little carbon nor too much. When this steel is honed there will be a clean surface at the edge that does not show a pattern and since the carbon has averaged between both kinds of steel and there are not any other significant alloying differences, it should function as a mono steel that has a pattern only where etched. I do not expect one layer or the other to function, or risk not functioning, differently from the other. During heat treatment, both layers achieve the same hardness so there is no myth of hard or soft layers in PW steel. That would require thermal controls that only a very few makers I know are capable of and it's not of any practical value in an edge regardless. It is this reason I choose the steel types I do.

Other alloys with other content differences may perform differently and this is where the maker has their say and the obligation to explain those differences when they become apparent. Some pattern welded steel makers choose to use pure nickel shim stock in the layers. Not only will that nickel layer not harden (it won't chip but will deform), but that layer also functions as a barrier to carbon migration and the carbon averaging that I expect, cannot occur to the same degree.

Using a strip of steel along the edge can be a practical blacksmith practice. It allows the smith to save money by using a smaller amount of high quality edge steel and lesser quality PW steels for the spine, or it prevents a weld line from being exposed right at the edge where it would be a fatal flaw to the cutting ability of the piece. Sometimes a flat core is sandwiched between two layers of lesser carbon, pretty, pattern welded steel (san mai construction). In the case of a kamisori, the tool maker might traditionally use a non hardening material like wrought iron and a small "tooth" of good quality steel for the edge.

There are a lot of variables to be controlled beside the steel choices. Heat history can be critical depending on steel type too. If the thin blade section is heated long enough, in the right kind of forge atmosphere, carbon can be lost to the environment and the decarburized area can be insufficiently removed during grinding. This might leave a section of the blade susceptible to crumbling or chipping. I have too many questions.