What is Creep failure and stages of creep


What is Creep

It is a failure mechanism that may occur in a material exposed for a prolonged length of time to a load below its elastic limit (Yield stress), the material increasing in length in the direction of the applied stress. The rate of deformation increases with, increase in the temperature so it is important to know the speed of deformation at a given load and temperature if components are to be safely designed for high temperature service. For this purpose, creep resistant alloys are developed. All metals and alloys are affected by the creep.

In metals, creep failure occurs at the grain boundaries to give an intergranular fracture. Figure 1 shows the voids that form on the grain boundaries in the early stage of creep.

Figure 1. Creep voids

Types of Creep Failures

There are several types of creep failure which can be characterized as follows:

Intergranular creep failure

This occurs after long-time exposure to temperature and stress. Early stages of long-term creep manifest as voids at the grain boundaries, these then subsequently link to form grain boundary fissures/cracks. As a result, there is little reduction in cross sectional area and a thick-walled fracture occurs. Non-destructive replication metallography is an effective means of determining the presence of long-term creep damage.

Furthermore, the platelets of iron carbide in the pearlite structure of carbon steels will thermally degrade to spheroidised iron carbide as a result of long-term overheating. Continued decomposition in plain carbon steels can result in total degradation to graphite plus ferrite. This degradation can also be detected using replication metallography.

Transgranular creep fracture

This type of fracture can occur in short-time creep failures. The ductility and reduction in area are usually large and much greater than at room temperature, producing a bulged, thin-walled fracture.

Point rupture fracture

At sufficiently high temperatures and low stresses, recrystallization during creep can remove microstructural creep damage. As a result, voids do not nucleate, and necking down to a point can occur.

Additions of chromium and molybdenum in steels can increase creep life. Mechanical or chemical cleaning is generally used to remove deposit build-up in boiler tubes which reduces the risk of local hot spots. An appropriate inspection program which includes monitoring wall thickness loss, microstructural degradation and creep damage is also an effective means of reducing the likelihood of creep failure.

Stages of Creep Failure in materials

Creep failure occurs in three different phases in materials when they are subjected to their creep temperature. In these creep stages material progressively loose their soundness and form creep voids. These creep voids under further loading will propagate in creep induced cracks.

  1. Primary creep: a rapid increase in length where the creep rate decrease as the metal work hardens.
  2. Secondary creep (Steady state): it is a period of almost constant creep rate and it is the period that forms bulk of the creep life of a component.

3. Tertiary creep: It occurs when the creep life is almost exhausted, voids have formed in the material and the effective cross-sectional area has been reduced. The creep rate accelerates as the stress per unit area increase until the specimen finally fails.

Different creep stages and their respective material conditions are shown in below figure 2.

Creep failure stages
Figure 2. Creep curve

Mechanism of Creep Failure

At elevated temperatures and stresses, much less than the high-temperature yield stress, metals undergo permanent plastic deformation called creep. Figure 3 shows a schematic creep curve for a constant load; a plot of the change in length verses time. The weight or load on the specimen is held constant for the duration of the test. There are four portions of the curve that are of interest:

Figure 3 Schematic creep curve
  • An initial steep rate that is at least partly of elastic origin, from point “0” to point “A” in above Figure 3.
     
  • This is followed by a region in which the elongation or deformation rate decreases with time, the so-called transient or primary creep, from region “A” to “B” of above Figure. The portion from point “0” to point “B” occurs fairly quickly.
     
  • The next portion of the creep curve is the area of engineering interest, where the creep rate is almost constant. The portion from “B” to “C” is nearly linear and predictable. Depending on the load or stress, the time can be very long; two years in a test and several decades in service.
     
  • The fourth portion of the creep curve, beyond the constant-creep-rate or linear region, shows a rapidly increasing creep rate which culminates in failure. Even under constant-load test conditions, the effective stress may actually increase due to the damage that forms within the microstructure.

Creep failures characteristics

Creep failures are characterized by:

  • bulging or blisters in the tube.
  • thick-edged fractures often with very little obvious ductility.
  • longitudinal “stress cracks” in either or both ID and OD oxide scales.
  • external or internal oxide-scale thicknesses that suggest higher-than-expected temperatures.
  • intergranular voids and cracks in the microstructure

Initial Creep Temperature of materials

The initial creep temperature of carbon steel, C-0.5Mo, 1.25Cr-0.5Mo, 2.25Cr-1Mo and stainless steel is shown in below table.

Carbon steel…………………..800oF
Carbon + 1/2 Molybdenum…………850oF
1-1/4 Chromium-1/2 Molybdenum……950oF
2-1 /4 Chromium-1 Molybdenum…….1000oF
Stainless steel………………..1050oF

Creep Failure vs. Fatigue Failure

Fatigue is a situation in which component is subjected to cyclic loading. Design Stress that is Endurance strength used in fatigue loading is much less than Yield strength and ultimate strength of material . 90 percent of machine components fail due to fatigue. For Example It is difficult to break a wire by stretching but if we apply a cyclic load and bend unbend the wire a number of times it breaks easily.
Creep is a situation in which a component experiences deformation under constant load with time as it is put into use. Best example to illustrate this is that electrical cables are taught(tight) when they are installed but after some time they experience sagging due to self weight.

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