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How to prepare a knife blade for heat treat

The hardening of a knife blade is the heart and soul of the knife. When done well, you’ll have a versatile tool for life.

When a knife blade is placed under load, stress is almost never uniformly distributed through the knife. Instead, it concentrates in certain areas where it can become severe enough to lead to a crack/s or fracture. Luckily there are a few things you as a knifemaker can do to prepare a blade for heat-treating so that the potential for warping and stress risers (crack initiation points) or other undesirable issues are minimised before heat-treating.

Below are a few pointers that you can incorporate into your knives straight away, but know that the overall design of a knife can also lead to stress risers/issues and those will need to be tested and addressed over time as the design evolves. [3]

Crack Formation

Chamfer Holes. All holes (bolster, handle pins, lanyard tubes, lightening, thumb flipper holes, etc) should be chamfered with a generous radius to better distribute tensile stresses. It also gives you a little flexibility in pin placement when fitting the pins. They should also not be placed too close to the edge, and not create a linear pattern that creates a natural path for cracks to develop.

  • Round hole, unchamfered: 100% (no reduction in maximum stress levels).
  • Round hole, chamfered 0.01″/0.254: 99.8%
  • Round hole, chamfered 0.02″/0.508mm: 90.4%
  • Round hole, chamfered 0.03″/0,762mm: 93.4% (reduction of 6.6% of stress around hole)

Round off edges. Like above, try to remove hard/sharp corners from the outer or inner edges of the knife blank. It may not be practical to do this around the handle, but also try to finish all edges lengthways with #120 or higher grit. Perpendicular scratch patterns, and especially those left by a #36 belt could be crack initiation points.

Round corners. Use fillets (radii) at the base of any sharp 90 degree corners to avoid stress concentrations. This includes slots for guards, shoulders of hidden tangs, file work & jimping any abrupt change in geometry that will likely cause stress concentrations. A drill bit can be an easy way of ensuring a small corner radius you aren’t using laser cutting services or have small needle files.

Logo stamps. Logo punches create abrupt charges of sections and have been shown to initiate cracks in blades. As such, it’s best to add markers marks after heat treating via negative acid etching, electro etching, or laser engraving.

Warping

Sharp Tools. Using bunt tools can cause excessive heat build-up which can lead to residual stresses in the blade and in the case of some air-hardening steels, work hardens the blade. Sharpen your drill bits and use cutting fluid. Use fresh belts when grinding. If there are stresses present before the heat treatment process, they can create internal stress issues that lead to warping.

Pre-grinding. If electing to pre-grind your bevels, cutting edge 45s, and or the scalloping of handles in preparation for tapering tangs, it is essential that it is done so evenly so that both sides have equal amounts of material removed.

Cutting Edge: Leave at least 1mm of thickness on the cutting edge if pregrinding. If you’re using blade steel that requires oil quenching, it’s best to leave the edge thicker due to the violence of the quench. If not the edge can warp in a wave pattern. This warpage cannot be corrected after it occurs.

Straight blanks: It probably goes without saying, but if the blade starts out straight it has a better chance of ending up straight. Please check your blades beforehand and take any bends out that are present.

Safety

Deburr: This is mainly for my safety, but if there aren’t burrs or sharp edges on your blades, please remove them.

By no means will these steps prevent 100% of the issue 100% of the time. Stuff happens. However, should you incorporate these into your process you’ll see far fewer issues in general, have fewer returns, and have happier clients.

References:

  1. How Stress Risers Lead to Broken Blades
  2. How Stress Risers Lead to Broken Blades (Blade Forum)
  3. How to Design Knives that Do Not Fail
  4. Defects and Distortion in Heat-Treated Parts
  5. When Metal Lets Us Down
  6. What a Good Heat Treatment Can and Cannot Do