We continue our discussion of normalizing. Part 3 of this series can be read here.
Applications for the Normalizing Process include: forgings, castings, weldments, some rolled products, bar and some tubular products, and steel sheet and strip.
One may ask the question, “What will happen to weldments after welding”?
If the two components to be welded are made of the same steel and the weld rod is of the same steel, then the heat-affected zone (HAZ) will be of the same steel. If the weld rod is of a different composition than that of the steel components, however, or if either of the two components are of a different composition, then the HAZ will be of a completely different composition and will result in a metallurgical variation of microstructures.
Stainless Steels and Tool Steels
Tool steels are usually supplied in the annealed condition from the supplier, or they can be normalized after forging. Most grades of stainless can be successfully forged. However, it is not recommended to normalize any of the stainless steel after forging. The following groups of stainless steel are forged.
- Austenitic grades
- Ferritic grades
- Duplex grades
- Martensitic grades
- Precipitation hardening grades
- Maraging steels
An interesting phenomenon occurs with the stainless steel groups (particularly the martensitic grades) while being forged. The mechanical strength will increase in the particular stainless steel, thus requiring more force than one would require to forge a plain-carbon or alloy steel. They are considerably more resistant to metal flow than the conventional steels. The more highly alloyed the type of stainless steel is, the more difficult it is to cause metal flow. In addition, care must be given to the soak time at temperature for forging and with consideration to grain growth.
With the austenitic stainless steels and the ferritic stainless steels, no phase transformation change is seen when the forging is on cool-down. The martensitic stainless steels, as with precipitation stainless steels, require a very slow and controlled cool rate after forging has been completed. The problem with the slow cool-down is that the steel surface should be protected.
Annealing is the post heat-treatment procedure to be adopted after forging. If the alloys are the low-carbon grades of stainless steels (e.g., austenitic or ferritic), the furnace atmosphere conditions need not be protective as far as decarburization is concerned.
For the martensitic and precipitation-hardening grades, it will be necessary to protect the steels against surface contamination. Typical atmospheres are:
- Argon – completely inert gas with no surface reaction on the steel whatsoever
- Hydrogen – Because hydrogen is a reducing atmosphere, any surface scale that forms will be reduced and the steel will be almost bright due to the surface reduction of oxide by the hydrogen.
- Salt bath – requires daily maintenance to keep the salt analysis at the appropriate level of chemistry
- Vacuum – cleanest surface finish of all the annealing processes with the least amount of surface-contamination risk. This method of processing will protect the steel against any potential for hydrogen embrittlement and against any surface nitrogen contamination. It will, however, give the slowest cooling of all of the annealing methods.