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Salt Bath HT vs. Vacuum HT of Tool Steels 1
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Thanks Maui- I realize this question was vague in nature, and was done so intentionally. i have a colleague looking for input into whether to stay with a salt bath process or look into vacuum heat treating for a variety of tool steels. The obvious benefits of salt bath are tight temperature tolerances of the bath, convection heating versus radiant heating, and it's ability to process more intricate designed components while minimizing distortion. I am saying you can get equivalent results with a vacuum furnace if you properly control heating and cooling. I think vacuum offers more flexibility in terms of processing various materials (gas quench or oil quench, multiple operations (normalize, harden can be done in one program, etc). Not to mention the environmental aspects.
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!
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!
Vacuum will produce a cleaner part, reducing extra ($) steps to clean or polish it. It really depends on the purpose and following process to the part.
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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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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Cycle Heaven.......
There are subtle microstructural differences when comparing salt bath vs vacuum heat treated for HSS(M2). My experience is from the late 80's, and both vac heating/quenching, and the nature of common HSS has changed since then. For instance, p/m HSS is fairly common, leading to more homogeneous m/s, no matter salt vs vacuum. At that time, we found that the m/s on salt bath heat treated was, how should I put it?--the work "tighter" comes to mind. The carbides were well distributed, and grain size remained small. Vacuum heat treated, while cleaner, same hardness, etc, resulted in a more "open" structure. Higher grain size, some carbides segregated to or near to prior austenitic(?) grain boundaries, some carbides "agglomerated". Despite these apparent problems, the customer's in-use tests showed little difference between them, and for environmental purposes, went with the vacuum ht.
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Whether your heat treat process utilizes salt bath, vacuum, controlled atmosphere, or fluidized bed furnaces, there is one important rule that must always be observed. There is no such thing as an acceptable short-cut in the proper heat treatment of a tool steel component. Failing to properly execute the required heat treating steps in order to save money on production costs will ultimately result in a tool that fails to adequately perform its intended function. Keep in mind as you read this that the tool that you make is only as good as the heat treatment that it receives. For this reason, an experienced commercial heat treatment shop with a solid reputation for quality work and attention to detail is highly recommended.
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:
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
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:
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
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Maui/Steve898/Pressed - thank you all for taking the time to provide the valuable feedback and information. your explanations have provided me an excellent baseline for pursuing this matter further. I'll look forward to additional correspondences in the future.
steve
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