This column is designed to help you troubleshoot your heat-treatment problem. It will show you how to be effective, efficient and logical in your troubleshooting and help you determine if the problem is process, material, design or control.
There are no hard-and-fast rules when it comes to troubleshooting the problem – only logical and progressive thinking. Remember, there are no bad ideas until they are proven to be bad ideas. Try to develop a thought process that, although it may take you outside the box, will help you be more effective in your troubleshooting.
“We have a problem” is an often-heard cry. This can come from the heat treater with his process or from the customer to the heat treater. How do we start to evaluate the problem and its root cause?
One of the most common problems in a heat-treat shop is that of distortion. Although there is potential to create distortion conditions in the steel, not all distortion is a result of heat treatment.
Any time a piece of steel is worked, machined, bent, twisted, rolled, milled, turned or ground, stress is being induced into the steel surface. The heat treater has enough to worry about with the phase changes that are about to take place.
Figure 1 demonstrates some of the factors that will contribute to distortion. The illustration is by no means complete. However, it indicates potential contribution factors to the problem of distortion.
Fig. 1. Factors contributing to distortion
Contributing Factors to Distortion
If the material is low on hardness or even totally soft, it is usually incorrect material. This can have occurred either by selection at the merchant or mixed at the purchaser’s operation. The method of steel identification can either be by spectrographic analysis or by spot chemical grinding into the sur-face of the steel to get into the “clean steel.”
If the component has failed completely, then the tree diagram shown in Fig. 2 should be considered. In the case of component failure, you should consider:
- Checking the quality of the furnace atmosphere
- Checking if sufficient stock has been removed prior to heat treatment to remove any mill decarburization
- Checking the salt analysis if salt-bath heat treated (salt could possibly be in a decarburizing condition)
- Checking for discoloration after pressure quench if the part has been vacuum heat treated (this is an indication of either a leak in the furnace at a seal or in the vessel itself)
- Checking for the possibility of contaminated quench gas
Fig. 2. Possible contributions to component failure
If the steel is only 3-6 HRC points lower than required after quenching, there is the probability of retained austenite being the root cause. The retained-austenite phase is usually found in the higher-alloyed steels, such as tool steels (O series, H series, HSS series and D series), but it can also be seen as a result of minimal control on carburizing steels.
The cause of the retained austenite can be attributed to the following conditions:
- Quenching from too high of an austenitizing temperature
- Quenching into too slow of a quench medium (slack quench)
- Excessive surface carbon in a carburized steel
Fig. 3. The interior components of a Rockwell hardness-testing unit
There are two remedies for the decomposition of a retained-austenite condition. The first is to cryogenically treat the steel, followed by tempering when the steel is back at room temperature. The second method is high-temperature tempering.
Another condition can be that of “spotty hardness,” which is hard and soft spots on the surface of the steel. This could be caused by the following:
- Vapor pockets during the quench
- Soot, if the furnace atmosphere is not effectively controlled
- Surface-chemistry changes if vacuum heat-treated
In the case of surface-chemistry changes, this could be caused by running the process at too low a vacuum level in relation to the austenitizing temperature. Outgassing of the surface elements could be occurring that would cause the change in surface chemistry.
Fig. 4. Possible causes for low hardness responses to heat treatment
Further Conditions that Contribute to Distortion
Remember to temper immediately when hand warm. Do not leave the steel and forget to temper it. Consider again cooling fast enough to form martensite, but this time cool down to a temperature above the martensite-start line. This will reduce the risk of distortion (austempering). Hold at that temperature and follow with air-cooling.
Quench cracks can occur as a result of too fast of a cooling rate on quench. Changes in the steel’s surface chemistry can cause cracking due to hardness gradients and phase changes. In order to re-duce the risk of cracking, consider reducing the cooling rate at quench.
Also, consider holding at just below the martensite-start temperature for a period of time and then al-lowing the steel to cool down to room temperature. This is known as marquenching.
Should you wish to acquire further information regarding an aspect of distortion that you are experiencing, I recommend searching the wealth of resources at www.industrialheating.com.