Temperature Limits – What Do They Really Mean?
Data sheets for most heat resistant alloys list a temperature limit. What does this temperature limit really mean? Can material always be used up to that temperature limit? The answer is: it depends.
A temperature limit for an alloy is its oxidation limit; the temperature at which the material can be used without too much oxide (scale) formation.
Oxidation is not the only limiting factor. The ability to resist creep is also an important consideration. Creep is metal stretching at a high temperature and can occur from loads well below the yield strength of the alloy. The concept is illustrated in the photograph below. A concentric ring was cut from each of four different alloys and welded to a plate. The assembly was placed into a furnace and held for an extended period of time. Two of the alloys did not stay round. They sagged to an oval shape. These two alloys experienced creep. The other two resisted creep and stayed round.
The important parameter is the creep rate, measured in % per hour. Minimum creep rate is a measurement used as the basis of engineering design for components at high temperature. It is the pressure required to produce a specific creep rate, most commonly 1% in 10,000 hours. In real designs, safety factors are built in. The higher the creep strength, the more weight a component can support.
Every alloy has a temperature above which there is not sufficient creep strength to be used to support any weight. Such temperatures are often well below the oxidation limit of the material. For example, stainless steel alloy 310 has an oxidation limit of 1090°C. It does not have sufficient creep strength to be used much above 980°C. In a load bearing situation, the practical limit for use would be 980°C. In some cases, a pipe might be used only to shield another component like a thermocouple. In this case, there is no structural load, so the material could be used all the way up to 1090°C.
Conversely, an alloy such as 253 MA® has an oxidation limit of 1090°C, but sufficient creep strength to be used up to 1140°C. Therefore in fully protective atmospheres (all surfaces of the metal are protected) or vacuum, 253 MA® can be used to 1140°C, when properly designed to hold a specific weight. Here the commonly specified temperature limit is 1090°C, but there are some good applications for use up to 1140°C.
Most published sources are going to continue using oxidation limit as the temperature limit. To understand the real temperature limit of your application, our team of metallurgists can provide you with technical support and help you identify the right alloy or alloys for your specific application.
It’s important to use the right temperature limit, not just the oxidation limit.
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