A tool is only as good as the material it is made from. Vise jaws can be made out of just about anything from brass or aluminum to copper or soft steel. The best material for vise jaws is Tool Steel.
There are several options for what kind of Tool Steel to use. I am an Old Timer. Based on my personal experience, I would choose S-7 or A-2. I believe S-7 is the best choice for Bench Jaws, but I use A-2. That’s because it’s easier to find and buy, and it comes in a wider selection of sizes in bar form. The most important reason I choose A-2 is because it is more stable after Heat Treating. Out of the oven, most Tool Steels are over 60 Rockwell, which is way too hard and brittle to use.
After the steel is cooled then it must be placed back in an annealing oven and re-heated to a certain temperature. For instance, S-7 to 500 degrees and A-2 to 1000 degrees, will take them both down to a tough but less brittle 54-56 R/C. This means when I weld, or use my torch flame close to A-2 jaws, I don’t worry about changing its hardness.
Here is a description of A-2 Tool Steel from Hudson-Metals.com:
“A2 Tool Steel is a versatile, air-hardening tool steel that is characterized by good toughness and excellent dimensional stability in heat treatment. A2 is intermediate in wear resistance between O1 oil-hardening tool steel and D2 high-carbon, high-chromium tool steel. A2 provides an effective combination of strength and toughness, tool performance, price, and a wide variety of product forms. APPLICATIONS: Punches and dies, chuck jaws, cutting tools for woodworking, tooling for plastic injection, dowel pins, hammers, industrial knives, and gauges.“
The original Wilton jaws built in the early 40’s to late 60’s were very hard. You can find some of these old Bullet Vises around with still sharp serrations even though they are over 50 years old. See the photo below with two old jaws from the 50’s and the new ones made by WiltonViseParts. The old ones can still can be used.
My Heat Treater and I tested several old jaw sets for their hardness and found they were a consistent 54-56 Rockwell Hardness. He believed they were made from an Oil Hardening Tool steel like O-1 which is an oil quench. That was a popular Tool Steel used at that time. A-2 and S-7 are both air cooled, and that is one reason they are so stable after heat treating. I have made jaws out of O-1, but found they warped more than I liked; about .007 over 5″. The A-2 shows no sign of warping, so that means the jaw face stays flat and will close parallel as it should.
When annealing Tool Steel, you can induce an inert gas to keep the jaws from oxidizing. My Heat Treater likes to use nitrogen. When nitrogen is used at 1000 degrees, and it adds a colorful pattern to the A-2 steel. Each jaw pattern is unique. See some examples in the picture below.
If the jaws are made from a good Tool Steel and heat treated to a proper hardness, then they should last the life of your vise. If you clamp a hard object like a car axle, or tighten down on a hard object like U-joints which are about the same hardness as your jaws, then the serrations will flatten or chip. Copper Caps not only protect the work piece, but also protect the jaw faces.
One thing I tell my customers is that they are buying a Premium set of replacement jaws, and if cared for, they should never need to buy more.