Hank Rearden
As I write this, keep in mind this was originally written for tool and die makers. (Machinist)
 in other words. As blacksmiths we also forge with tool steel as well as heat treat tools and dies. Here are few things I found interesting and I think you will as well. I'm going to condense the subject to touch points I didn't know.

One cause of cracking and distortion during heat treatment is a faulty design. Steel can fail due to two types of force combing to cause the failure. 1. The internal strains created during fabrication and heat treatment.  2. The external forces of service. To illustrate this consider the corner of a cube of steel. A point located closest to the corner has less mass and will heat and cool and at different rate than a point located in the center of one side of the cube. This is exaggerated by quenching.

This difference is referred to as temperature gradient. Temperature gradient is the rate of variation in the temperature of the metal over a given unit distance. To better understand this consider a point near the corner of the cube. It's temperature is 300 degrees. The point toward the center is 700 Degrees. A difference of 400 degrees. Now let's say those two point are one inch from each other. Now divide the inch into quarters. The temperature will rise if measured at each quarter inch mark if measure from the corner to the center of the cube.

If we have a piece of steel we take from the forge and it's glowing a uniform  bright cherry red of 1450 degrees. We assume that temperature is consistent through out the steel.

So now lets consider quenching. You can use air, oil and water to quench. The rapid change created by quenching in oil will be less than in water. Air will be less than oil.  So if it's cracking or distorting with one of the types of quench it may work fine with a different type quench.

Another example the book gave was that of a cone shaped object. When quenched the point will cool at a faster rate than the heavier and larger end. Because we have more heated mass it will cool at a slower rate.

Another term in this chapter is re-entrant angle. Imagine the letter L as three dimensional. Now pick a spot in the center of the corner on the inside angle.  This area has heat 3 times that of the bottom outside corner of the L. This sharp angle creates a problem with uniform cooling during quenching. It also lends to the possible creation of air pocket causing soft spots. This creates internal stresses in the material and is prone to failure.

To help alleviate these stresses soften the re-entrant angles by smoothing the sharp square angles with a radius. You can also remove material from of high mass by drilling or punching.

The book goes into more detail and examples. This was on the tool designs with grooves, ribs, teeth and keyways. The principles in theory can be adapted to punches, hammers anvil hardies and blades a blacksmith forges.

I hope I conveyed the ideas and welcome questions and comments.



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