Salt Bath HT vs. Vacuum HT of Tool Steels
Salt Bath HT vs. Vacuum HT of Tool Steels
(OP)
Looking for information comparing the pros and cons of salt bath and vacuum heat treating of tool steels.
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Salt Bath HT vs. Vacuum HT of Tool Steels
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RE: Salt Bath HT vs. Vacuum HT of Tool Steels
Maui
RE: Salt Bath HT vs. Vacuum HT of Tool Steels
I'd welcome any practical comparisons of results from any tool steel processed in both a salt bath furnace and a vacuum furnace. The rationalization for one process versus the other may not be appropriate for all tool steels but I'd like to get a general sense of how they compare.
Thanks for your input!
RE: Salt Bath HT vs. Vacuum HT of Tool Steels
Some vacuum furnaces have the capability of high quench rates. This is helpful depending on the particular steel and if your current salt bath processor is experiencing retained austenite in your parts.
Heat treaters that use the same trays for hardening and other processes like nitriding will cost you less and be processed quicker. There wil be less handling and the parts will go from one unit to the other, quickly.
I think there are more computer controlled vacuum furnaces than not. This possibly relates to stored recipes, repeatability and record keeping. I think there are more non-computer controlled salt furnaces than not. I could be wrong on these two points but that is what I have seen in all the plants I have evaluated.
Now go finish that arguement.
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RE: Salt Bath HT vs. Vacuum HT of Tool Steels
RE: Salt Bath HT vs. Vacuum HT of Tool Steels
One of the important differences typically observed between salt bath and vacuum heat treating is hardening response. Salt bath heat treatment usually results in one to two points higher Rockwell C hardness than an equivalent vacuum heat treatment process for the same tool steel. This is primarily due to the uniform temperature distribution that is attained in a salt bath furnace and the rapid quench rate that is achieved in either salt or oil in comparison to vacuum quenching. For example, tabulated values of hardening response for CPM Rex T15 are quoted in the following link:
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These hardness values are based on salt bath heat treatment (even though this is not explicitly stated in the data sheet itself). In note A beneath the hardening data it states, “Results may vary with hardening method and section size. Salt or oil quenching will give maximum response. Vacuum or atmosphere cooling may result in up to 1 to 2 HRC points lower”. The amount of time it takes to quench the component down to about 1000 F usually determines the hardening response of the material. The general relationship is this: the faster the quench, the harder the part. The Bar rating indicates the severity of the quench that a vacuum furnace is capable of achieving. A 2 Bar vacuum furnace is the minimum quench capability that is normally recommended for heat treating tool steel and high speed steel components. Commercial furnaces with a 10 Bar capability are not uncommon, and I am aware of one furnace with a 20 Bar capability. These vacuum furnaces are able to quench parts rapidly enough that they can rival the hardening response obtained from either salt bath or oil quenching.
Under the proper operating conditions, salt bath heat treatment can be used to process parts that do not require any additional grinding after heat treatment, although this is not usually done in practice. Most components that are subjected to salt bath heat treatment have a specific amount of oversize that is built into the part. Finish grinding is done after heat treatment is completed in order to bring the part dimensions into tolerance. This additional step removes any carburized or decarburized salt attacked surface layers that are present, and assures that the working surface of the part is properly finished. Careful maintenance of the salt bath composition and furnace operating conditions will minimize the occurrence of carburization or decarburization, but some amount of either can be present. For parts where the heat treated surface will be used as the working surface of the finished tool, vacuum heat treating is the preferred method. Vacuum hardening often requires a slightly higher soak temperature and longer soak time than salt bath hardening due to the uncertainty in determining when the component actually reaches the soak temperature. Similar problems can also occur with atmosphere controlled furnaces. As an example of this, refer to ASTM A600 92a, page 354. For T15, which appears at the bottom of the column in Table 3, the recommended austenitizing temperature for salt bath heat treatment is 2240 F, while for a controlled atmosphere furnace it lists an austenitizing temperature of 2260 F. The increased hold time at elevated temperature can result in increased grain growth, which can lower the toughness of the resulting component. So several factors need to be considered in determining what method of heat treatment is most appropriate for your specific application.
Maui
RE: Salt Bath HT vs. Vacuum HT of Tool Steels
steve