Can you talk about what causes distortion in heat treating and how to avoid it?

This is a complex subject, but listed below are a few of the distortion issues people have reported. Of these factors, those resulting from transformation during heating and cooling are often ignored. These rates produce internal stress and strain (induced by volumetric changes due to transformation) and result in localized deformation and general part distortion.

Part design/geometry, steel production methods and steel quality are important as well. The shape of the part, or the "design geometry," dictates to a large extent the post heat-treat distortion results. Most often this parameter is fixed for a given application. Steel production methods and steel quality, including cleanliness (inclusion level and distribution), Jominy (hardenability over a given range), grain size, and tramp-element percentages also play a significant role.

When a precision-machined gear is put in a furnace, raised to austenitizing temperature, carburized for an extended time and then quenched to produce a martensitic microstructure, distortion is unavoidable. There are, however, ways to minimize and/or manage the amount and type of distortion, making it more predictable from part to part and lot to lot. As a result, machining processes can be developed that will consistently yield gears meeting the design requirements, both dimensionally and metallurgically.

A common misconception by designers unfamiliar with these effects leads to adding more stock and specifying a deeper case depth in an attempt to compensate for part distortion. Not only does this add cost, but also post-grinding operations will result in the unintentional removal of some or all of the "depth of high hardness" (that is, that part of the case where the hardness is 58 HRC). Thus the best part of the case, whether atmosphere or vacuum carburizing is specified, is being sacrificed.

Irrespective of the effective case depth specified, for a given geometry and loading arrangement, heat can only be removed from the part by the quench medium at a fixed rate. This means that even though the depth of 50 HRC may be greater if a deeper effective case depth is specified, the depth of the surface layer where the hardness is 58 HRC is not proportionally deeper and, as a result, excessive grinding sacrifices surface hardness.

Causes for Distortion General
  • Failure to normalize the material
  • Improper annealing
  • Excessive stress from machining (drilling and milling are common sources of induced stress.)
  • Surface tearing or burnishing (creates stress risers on the part)
  • Holes, slots or varying section on a part (These areas will quench faster causing differential part cooling.)
  • Part too long, high length/diameter ratio (As a rule, a ratio over eight-to-one results in distortion.)
  • Excessive case depth of case hardening (Distortion tends to increase with deeper case depths.)
Heat Treat Distortion Effects

The effects of heat-treat distortion on different blueprint dimensions:
  • Outside diameter grows (requires finishing operation)
  • Inside diameter generally shrinks (requires finishing operation)
  • Thin tubing generally adopts an oval shape (concentricity is lost)
  • Length may shrink or grow based on part geometry
  • Long part bends unevenly
  • Cross holes & slots shrink and may cause stress riser
Distortion Cost Factors

In addition to the furnace, other factors affect the heat-treatment costs include:
  • Scale and debris (very difficult and costly to clean up)
  • Cleanliness
  • Specific carburize area or section (When specific areas on the part require carburizing, the remaining areas must have the carbon removed after carburizing.)
  • Lot size (i.e. full or partial furnace load)
  • Micro cracks and high stress-riser areas