Proprietà delle alte temperature Acciaio inossidabile

Stainless steel have good strength and good resistance to corrosion and ossidazione at elevated temperatures. Stainless steel 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 scambiatore di calore, super-heaters, caldaia, feed water heaters, valves and main steam lines as well as aircraft and aerospace applications.

Figure 1 gives a broad concept of the hot strength advantages of stainless steel in comparison to low carbon unalloyed steel. Table 1 shows the short term resistenza alla trazione e resistenza allo snervamento vs temperature. Table 2 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 precipitazione, or embrittlement. Softening or loss of strength occurs in the 300 series (304, 316, etc.) stainless steel 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).

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 (Titanio 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 acciaio inossidabile duplex 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 durezza e resistenza alla trazione at room temperature, but retains its desirable proprietà meccaniche at operating temperatures.

Table 1 Short Term Resistenza alla trazione vs Temperatura (allo stato ricotto, tranne che per il 410)

* trattato termicamente by oil quenching from 1800° F and tempering at 1200° F

Table 2 Generally Accepted Service Temperature

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 steel and the scale. As a result of this scaling and cracking, there is a greater deterioration of the superficie 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.

Riferimenti correlati:
Uso dell'acciaio inossidabile in condizioni di alta temperatura, vedi tabella
Tubo in acciaio inox resistente alla corrosione
Resistenza alla corrosione dei tubi in acciaio inossidabile
Effetti della temperatura sulla resistenza dei metalli
Tubi in acciaio inox per alte temperature
Tubi in acciaio inox per alte temperature
Acciaio inossidabile con proprietà ad alta temperatura
Tubi in acciaio inox resistenti al calore
Tubo in acciaio inox saldato
U-bend Stainless Steel Tubes
Tubi per scambiatori di calore
Duplex Stainless Steel Tubes
Tubi per caldaie, tubi per condensatori
Corrugated Seamless Stainless Steel Pipe Tube
Tubi in acciaio di precisione senza saldatura DIN 2391
EN 10305-1 E215 E235 E355 Tubi di precisione in acciaio senza saldatura Tubi di precisione
Non Acid-resisting Stainless Steel Tube
Tubi in acciaio inox a ricottura brillante
Tubi per alte temperature e standard
Proprietà tecniche dell'acciaio inossidabile modificato ad alta temperatura
Acciai inossidabili resistenti al calore e acciai inossidabili resistenti alla corrosione - acciai per valvole, leghe di ferro Super leghe
Rischi di scintille in atmosfere con gas esplosivi