Stress relief pre-heat treatment vs. straighten/flatten post-heat treatment

While the residual stress system can be modified by removal of material, further forming of the raw material, and heat treatment processes, modification invariably leads to distortion or cracking. 

“Distortion may be rectified by straightening or flattening operations. However, there is always
a risk of breakage or cracking with such operations,” cautions Roger Haw, Managing Director of contract heat treatment specialist Flame Hardeners Ltd.

Problems associated with changes of residual stress systems are often found in the heat treatment of rollers, shafts, and pins – together
with items such as gear rings, bearing rings, guide bars, machine tool beds and press brake tools.

Modern machining techniques have led to increased rates of metal removal and, in many cases, use of bar stock to produce complex machined parts from which
50 percent or more of the original material volume has been removed. 

“Cases have been seen where, for the benefit of expedient delivery of cylindrical items, such as rollers and piston rods, available bar stock has been used which can be 20% greater than
the diameter required. Gear shafts, which may have been made from forgings some years ago, are now manufactured from bar stock and diameters of the length of a shaft can vary by as much as 200 percent,” Haw elaborates.

Read the full article in the July issue of DPA.