Causes

  • Initial steel carbon concentration
  • As the carbon potential increases, both transformation temperature of the Ms and Mf will be lowered (i.e., lower than room temperature).
  • On cooling down to room temperature, a significant amount of austenite will have transformed to fresh martensite, which has the largest molecular volumetric size. Some austenite will remain (hence the term retained austenite).
  • The residual austenite will change naturally over time. This means that the steel will begin to become slightly harder due to new martensite developing.
  • The second factor will be that growth in size will occur because of the new fresh martensite (naturally transforming to martensite), .

 

Conclusion

After reviewing photo micrographs, cross-sectional hardness traverse results and process documentation for process times and atmospheric conditions, the investigation revealed:

  • Misunderstandings in the knowledge of dew-point testing and control of process atmosphere parameters
  • Limited knowledge of the function and control of natural gas and air ratios for the endothermic generator
  • Limited knowledge and understanding of the influence of material composition
  • No knowledge of the value of the material test certificate. The cert was only read and filed away.
  • No incoming material physical examinations were conducted, such as material hardness testing.
  • Minimal understanding of the transitional changes that occur with the application of heat to steel.
  • They told me: “We have always done it that way.”

The process of carburizing is a very sensitive one. The company changed over to the use of the oxygen probe for real-time atmosphere control. It was further recommended to use a backup check system such as shim analysis. This is not a real-time method – it is generally about 1 hour behind the oxygen probe – but it does give almost a belt-and-suspenders to the accurate and repeatable analysis of the oxygen probe. This was followed by stricter and better control of the quench-medium temperature and its ability to stay within 20-30°F to ensure an almost repeatable quench condition.

We made up a wooden box with a steel internal open container to demonstrate (inexpensively) that cryogenics work with the decomposition of retained austenite. The internal container was insulated with a refractory insulating blanket. The inner container was filled with a suitable low-temperature solidification liquid. The crushed dry ice was added to reduce the liquid temperature.

This is now standard practice for the company, which has improved its metallurgical quality, improved the product's performance and increased its sales.