High Temperature Creep Rates and Creep Rupture

Creep-Rupture

Why short term yield and tensile properties may not be useful at high temperatures.

Metals behave much differently at high temperatures than they do near room temperature. If a metal bar is loaded to just below its yield strength at room temperature, that load can be left there practically forever.

Now let’s say that this metal bar is loaded, again keeping the stress below the yield strength—while it is glowing cherry red, 1500°F. A very small amount of deformation will occur at first (first stage creep). Then that metal bar will begin to stretch, but very, very slowly. It will keep on stretching for hours, weeks, maybe years, until it finally breaks in two. All this, when it wasn’t even loaded up to the yield strength (as measured by a short-time tensile test).

 

Creep

The rate, or speed, at which the metal is stretching, in % per hour, is called its “creep rate”. Creep rate is expressed as percent deformation per hour. For some period of time the creep rate is more or less constant. This is the “minimum creep rate”, or “secondary creep rate”.

The minimum creep rate is used as one basis for design at high temperature. That

is, at high temperature one must assume that the metal is going to creep, or deform, to

some degree. This is true even for light loads. Theoretically, the designer might settle on an acceptable amount of creep deformation over the projected life of the equipment. He would then pick his design strength based on the speed of deformation acceptable in his application. In the furnace industry one design criterion is the stress required for a minimum creep rate of 1% in 10,000 hours, or 0.0001% per hour.

Design stress may be set at some fraction of this number. The ASME code uses for one of its criteria 100% of the extrapolated stress for 1% in 100,000 hour mcr, or 0.00001%/hr.  The other measure of creep, and the one used in Europe, is “total creep”. That is, the stress required for the specimen to actually stretch a total of, say, 1%. Minimum creep rate data and total creep rate data are not interchangeable. People tend to have rather strong feelings about one or the other creep measurement, so whenever possible we provide both minimum creep and total creep data.

 

Rupture

“Rupture Stress”, or “Creep-Rupture Strength”, is reported as both a stress, and a number of hours. It is the stress required to completely break a specimen within a given amount of time. In the furnace industry another common criterion for setting design stresses is to use some fraction of the stress that would result in rupture at 10,000 hours. ASME design criteria is conservative  and uses 67% of the extrapolated 100,000 hour rupture stress, or 100% of the extrapolated 1% in 100,000 hour minimum creep rate, whichever is less.

creep behavior

Creep strength is more important than rupture strength. For example, at 1800°F the alloys RA330, RA309 and RA310 all have comparable 10,000 hour rupture strength,

about 560--660 psi. However in service an RA330 muffle or retort can retain its shape for years, whereas one of RA309 or RA310 would collapse. The reason is, these stainless heat resisting grades have only 40—55% of the creep strength of RA330 at 1800°F.

Normally we expect the strongest alloy to do the best job. This does depend on how that strength is achieved. For example, RA333 and RA 253MA are strengthened by various alloy additions, with a medium-fine grain size. As a result, they all have good to excellent thermal fatigue resistance in quench applications.