Metals Science Research and Consulting


by Dr. Stan T. Mandziej

 

Accelerated creep test


“The creep of steels and welds is a form of plastic deformation, causing at elevated temperature transformation of microstructure and decrease of strength, leading to rupture”. (R.W.K. Honeycombe: The Plastic Deformation of Metals. London, E. Arnold Publ, 1984).

“The problem of defining creep mechanisms, even in single crystals, is still far from accomplished”. (R.E Reed-Hill & R. Abbaschian: Physical Metallurgy Principles, Boston, PWS-KENT, 1992).

 

 




Creep voids form preferentially at boundaries of grains in which evidences of oriented slip occur. On walls of creep voids z-shaped features appear.

 






Crept steel, when fractured at low temperature, reveals on surfaces of larger creep cracks characteristic pattern of crisscrossing parallel lines.

 




In the second stage of creep, in the substructure of steel dominate parallel arrays of  a/2<111> screw dislocations, often gliding on perpendicular {110} planes.

This correlates with the above fracture.

 



So the most probable micro-mechanism of the creep crack formation is by piling-up the dislocations to form the voids and then coalescence of the voids.

 

To reproduce the sequence of microstructural events during creep, a thermal-mechanical fatigue procedure was developed on Gleeble physical simulator. The procedure was initially intended to generate data of creep strength and creep life for weld metals and weld joint HAZs, which in general are non available.

 

Samples for the ACTest and their mounting in Gleeble’s pocket-jaws assembly.

 

A sample after the test and crack in the middle of it; then uniformly-heated zone (UHZ) and transient zone (TZ) in the tested sample; the TZ is 1.0—1.5 mm wide, depending on grade of steel and span between the mounting "cold" copper jaws.

 

A result of the ACTest: strain-time graph; duration of test about 6 hours.

Recorded on Gleeble stress-time and strain-time data allow to calculate the creep lifetime of tested material, in particular of  weld metal or heat-affected zone of the base metal.

 

Crisscrossing a/2<111> screw dislocations, at half-life in ACT, representative to the second stage of creep (left picture) and the surface of ACT fracture (etched) with visible steps created by piled-up dislocations (right picture). 

 


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Full article related to this subject, entitled: 
 "Physical  Background and Simulation of Creep in Steels" 
is available from: amatemlab@gmail.com, upon request. 

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The procedure of evaluating creep lifetime from the Gleeble data is proprietary of A.M.A.