We continue David Pye's blog series on the causes and effects of distortion. Part 1 can be found here.
 

Material

Multiple material-related issues affect distortion. These include:

  • Composition
  • Cast variations
  • Micro-segregation (manganese and sulfur)
  • Gross segregation (chrome)
  • Decarburization
  • Surface oxidation (scale)

The heat treater often hears, “We give you AISI 4140, and every time we get a variation in hardness and a different hardness value that what we got previously.” While a valid statement, it is unfair to blame the heat treater. Consider AISI 4140 and its specification and variation (see Table 1).

If we consider the variances that are present in steelmaking, it is difficult to produce a one-process-temperature-suits-all heat-treatment procedure. This is why there will always be some hardness variance in the as-quenched hardness. It can never be entirely repeatable and consistent with a single austenitizing temperature.

 

Other (Manufacturing) Factors

Figure 2 illustrates some of the various factors that “cause” residual stress and distortion. Obviously, the heat treater has no control over the machining environment, which can cause residual stresses. Residual stresses cause shape change when they exceed the material yield strength, which occurs on heating when the strength properties decline.

Figure 3 displays the potential for distortion that can occur as a result of bending or forming due to the grain-flow orientation. Once heat is applied to the bend, it will try to open outward and relieve itself of the forming stresses.