Carpenter

Fabricating Carpenter Stainless Steels

June 2006

Forging Carpenter Stainless Steels

In all metalworking operations stainless steel can be easily worked when the characteristics of these alloys are understood. Stainless steels have good inherent forgeability, but there are important differences from the carbon and low-alloy steels.

Most importantly, stainless steels are much stronger at forging temperatures and thus require greater force or more blows under a hammer than is required for leaner alloys. The high temperature alloys are even harder and more resistant to flow in forging operations. 

All stainless steels have much lower thermal conductivity than ordinary steel—thus the heat penetrates the steel more slowly. The best results are obtained in a muffle or semimuffle type of furnace with pyrometer control. Keep open flames away from the steel.

As shown in the table, the forging temperature depends upon the type of steel—austenitic, martensitic, ferritic, duplex or precipitation hardenable, with a few special cases. There is no simple rule to follow for thermal handling on either heating or cooling. The suggested forging temperatures should be attained by heating in furnaces held at those temperatures (all temperatures are furnace temperatures, not die temperatures). The furnace must not be run excessively hot and the steel withdrawn "on the fly" as it rushes up to the forging heat. This gives a wash heat on the surface and a cold center. 

Hold the heating furnace steady at the proper forging temperature and no hotter; allow the steel to soak out a little before withdrawing, and it will flow readily under the dies. In order not to slow down the forging operation and still run the furnace at a "slow" heat, more bars or billets can usually be heated at one time.

Most grades are subject to rapid grain growth at the forging heat. If all parts of the steel are thoroughly forged after heating, the grain structure will be refined again. If some parts of the forging get little reduction under the hammer, care must be exercised to limit grain growth by avoiding a long soak at temperature.  

Surface preparation of forging bars and billets is generally more critical for stainless steels for several reasons. One example is the aircraft industry, which demands close tolerances for weight economy. This allows little or nothing for removing defects from finished parts. Any forging job will cost less if no defects must be removed because of poorly prepared stock.

Lastly, stainless steels require special heat treatments after forging to obtain best corrosion resistance and mechanical properties. (See the chart.) Briefly, the austenitic, ferritic and duplex grades should be annealed for optimum corrosion resistance; the martensitic grades are air-hardening and require slow cooling after forging plus subsequent annealing to prevent cracking; and the precipitation hardenable grades require a solution anneal for optimum aging response. 

Carpenter practices have been perfected for developing stainless steels that have optimum forgeability as opposed to, say, optimum machinability. The factors that contribute to good inherent forgeability in Carpenter stainless steel are as follows:

1.       Controlled melting process for sounder centers, cleaner metal and less center segregation. 

2.       Balanced analysis for better metal flow, reduced hot shortness, and less in-process preparations.

3.       Rare earth additions to highly alloyed austenitic grades such as 20Cb-3^® stainless for reduced hot shortness and better yields. 

Every metal fabricator who hot-works steels and alloys knows how important it is to determine the best temperature range for forging each grade. The more narrow the forging range, the more critical the problem becomes.

Many tests used to predict hot-working temperature ranges are helpful in that they offer a rough measure of forgeability over a given range, but they do not give specific values. This has forced forgers to rely on approximate temperatures which, in many cases, are not the best ones for the material being worked. 

Hot tensile ductility is often used to determine the forging temperature range for a given alloy. Evaluation is performed using a Gleeble thermomechanical testing unit. The main feature of the unit is the ability to reproduce any desired thermal cycle on a test specimen via resistive heating.

Whereas inherent forging quality is melted into stainless steels, there is another equally important aspect to Carpenter forging quality: mechanical forgeability. This includes factors that contribute to soundness: 

1.       Disc inspection and sonic inspection of in-process billets and finished forging billets.

2.       Adequate surface preparation both on in-process billets for manufacturing forging bars and also final surface preparation of forging bars and billets 

3.       Quality control upset forging tests conducted on critical forging bar items.

Ask your Carpenter representative for additional information on Carpenter stainless steels for the forging industry.