The most general definition of creep is this: creep is a diffusion-controlled, thermally activated process that results in slow, continuous plastic deformation. For most materials that are tested at room temperature, the plastic strain is a function of the applied stress, but not a function of time. At relatively high temperatures 0.5Tm > T > 0.3Tm where Tm is the solidus temperature, metals and ceramics usually begin to exhibit time-dependant plasticity. This type of behavior is called creep, and it is usually the limiting factor in the selection of materials for use at elevated temperatures. For example, suppose we apply a stress of 10,000 psi to a bar made from structural steel and measure the elongation. If we leave the bar under load overnight, there won’t be any significant change in the elongation the following morning. But if we place the same specimen in a furnace at 1000 F and run the experiment again, we would find that there would be a measurable increase in the elongation the next morning. This is because the material creeps at 1000 F. It undergoes slow, continuous plastic deformation.

There are in general three stages to creep deformation. During the initial stage, called primary creep, the strain increases rapidly with time. During stage II, called secondary creep, the material enters a steady state and the strain increases steadily with time. During stage III the strain increases rapidly with time until failure occurs. The useful life of a material is usually spent in stage II, so that secondary creep plays a dominant role in determining the lifetime of a component. An empirical formula for the steady state creep rate is given by,

     Creep Rate = A*[(stress)^n]*exp{-(Q/RT)}

where A is the creep constant, n is the creep exponent (which usually lies between 3 and 8), Q is the activation energy, R is the universal gas constant, and T is the absolute temperature. If we know A, n, and Q for a specific material, then we can calculate the strain rate at any temperature and stress. For a fixed temperature, this equation represents what is known as power law creep.
        
There are two distinct mechanisms for creep: dislocation creep and diffusional creep. Both are limited by the rate of atomic diffusion, so both follow an exponential temperature dependence according to Arrhenius’s Law. When a dislocation encounters an obstacle, it slows down. But if the temperature of the material is greater than about 0.3Tm, atoms may diffuse around the obstacle quickly enough to “unlock” the dislocation. This makes it much easier for the dislocation to move past the obstacle. The motion of these “unlocked” dislocations under the applied stress is what leads to dislocation creep.
    So how does this happen? When a dislocation encounters a hard precipitate, it can’t glide upwards to clear the obstacle because that would force it out of its slip plane. But if the atoms at the bottom of the half-plane are able to diffuse away, then the net effect is that the dislocation climbs upward. The applied stress acts as a mechanical driving force for this to occur. After the dislocation climbs high enough, it can clear the precipitate and then continue to move along its slip plane. After a short time it will encounter another obstacle, and the whole process repeats itself. This explains the slow, continuous nature of dislocation creep. I've probably supplied more than you needed, but does this answer your question?


                                            Maui

Thanks much, maui.  That was very helpful.  Can you go a little further and explain creep embrittlement (as it pertains to low chrome metals)?  What is the difference between creep and creep embrittlement?

I've never heard of creep embrittlement. Specific alloys undergo a loss of toughness if they are sujected to tempering within a certain temperature range. If the tempering temperature is high enough, this may coincide with the initiation of creep. What alloy system are you interested in, and what temperature range are you using?


                                         Maui

Are you referring to temper embrittlement???
If so, Temper embrittlement is a phenomenon by which upon prolonged exposures(as in service) at about 450 Deg C, a marked increase in the DBTT is observed. The principal reason for this condition appears to be the segregation of P,As,Sb and Sn at the austenite grain boundaries. Typically 2 1/4 Cr 1 Mo steels are known to be susceptible to temper embrittlement(Is this because of the higher Mo as compared to other Cr Mo steels??- I think so, I am not sure)
To ensure that the steels resist such embrittlement, restrictions are placed on above elements for both base metals and weld metals. For e.g. to qualify a LA steel plate like say SA387 Gr22 Cl 2 for such service, very stringent requirements on the material chemistry are to be met. These are defined as the J and X Embrittlement factors with
J Factor= (Si+Mn) x (P+SN) x 104 </=160(elemnts expressed in wt%)
and
X factor=(10P+5Sb+4Sn+As) </=15 (elements expressed in ppm)In addition to these tests the material or welds are subjected to typical prolonged step cooling simulated heat treatment cycles and the impact curves are plotted for the normal and post step cooling. The shift in the DBTT is then studied and reported. The DBTT and the limits for the shift in temperature aree normally defined as well.

Thanks and regards
Sayee Prasad R
Ph: 0097143968906
Mob: 00971507682668
email: sayee_prasad@yahoo.com
The whole of science is nothing more than a refinement of everyday thinking!!!  

Maui,
A star for your replies. Are you in the teaching profession or into research?? A couple of replies before this from you on other forums and this reply as well are made with so much clarity and patience as would be from a Professor/Lecturer.

Thanks and regards
Sayee Prasad R
Ph: 0097143968906
Mob: 00971507682668
email: sayee_prasad@yahoo.com
The whole of science is nothing more than a refinement of everyday thinking!!!  

Thank you for the compliment sayeeprasadr. You were correct on both guesses. I work as a Senior Metallurgist at a specialty steel mill, and I also teach a materials course at Syracuse University part time. I've taken notice of several of your insightful replies in other threads. Are you a professional engineer/metallurgist?


                                           Maui

Maui
I am a metallurgist (by choice) and a welding engineer. Been linked with welding since the day I walked out of engineering college and have worked with all functions, namely sales, marketing, production and QC in welding related job functions. Currently working as a welding engineer for an offshore structures and platforms fabricator in the Middle East.

Thanks and regards
Sayee Prasad R
Ph: 0097143968906
Mob: 00971507682668
email: sayee_prasad@yahoo.com
The whole of science is nothing more than a refinement of everyday thinking!!!