HIGH TEMPERATURE PROPERTIES

Stainless steels have good strength and good resistance to corrosion and oxidation at elevated temperatures. Stainless steels are used at temperatures up to 1700° F for 304 and 316 and up to 2000 F for the high temperature stainless grade 309(S) and up to 2100° F for 310(S). Stainless steel is used extensively in heat exchangers, super-heaters, boilers, feed water heaters, valves and main steam lines as well as aircraft and aerospace applications.

Figure 1 (below) gives a broad concept of the hot strength advantages of stainless steel in comparison to low carbon unalloyed steel. Table 1 (below) shows the short term tensile and yield strength vs temperature. Table 2 (below) shows the generally accepted temperatures for both intermittent and continuous service.

With time and temperature, changes in metallurgical structure can be expected with any metal. In stainless steel, the changes can be softening, carbide precipitation, or embrittlement. Softening or loss of strength occurs in the 300 series (304, 316, etc.) stainless steels at about 1000° F and at about 900° F for the hardenable 400 (410, 420, 440) series and 800° F for the non-hardenable 400 (409, 430) series (refer to Table 1, below).

Carbide precipitation can occur in the 300 series in the temperature range 800 – 1600° F. It can be deterred by choosing a grade designed to prevent carbide precipitation i.e., 347 (Cb added) or 321 (Ti added). If carbide precipitation does occur, it can be removed by heating above 1900° and cooling quickly.

Hardenable 400 series with greater than 12% chromium as well as the non-hardenable 400 series and the duplex stainless steels are subject to embrittlement when exposed to temperature of 700 – 950° F over an extended period of time. This is sometimes call 885F embrittlement because this is the temperature at which the embrittlement is the most rapid. 885F embrittlement results in low ductility and increased hardness and tensile strengths at room temperature, but retains its desirable mechanical properties at operating temperatures.

Table 1
Short Term Tensile Strength vs Temperature
(in the annealed condition except for 410)

* heat treated by oil quenching from 1800° F and tempering at 1200° F

Table 2
Generally Accepted Service Temperatures

It may seem to be illogical that the "continuous" service temperature would be higher than the "intermittent" service temperature for the 300 series grades. The answer is that intermittent service involves "thermal cycling", which can cause the high temperature scale formed to crack and spall. This occurs because of the difference in the coefficient of expansion between the stainless metal and the scale. As a result of this scaling and cracking, there is a greater deterioration of the surface than will occur if the temperature is continuous. Therefore the suggested intermittent service temperatures are lower. This is not the case for the 400 series (both ferritic and martensitic grades). The reason for this is not known.