Transition temperature

Transcription

1 Transition temperature Different criteria are used to determine the transition temperature, depending on the purpose of the application. Various criteria of transition temperature obtained from Charpy test 1) T 1 transition temp is the Temp at which fracture is 100% ductile (fibrous). 2) T 2 transition temp is the Temp at which fracture is 50% cleavage and 50% ductile. 3) T 3 transition temp is the Temp at the average energy absorption of upper and lower shelves. 4) T 4 transition temp is the Temp defined at C v = 20J. 5) T 5 transition temp is the Temp at which fracture is 100% cleavage. Note: FTP is fracture transition plastic

2 Metallurgical factors affecting DBTT curves The shape and position of the DBTT curve is important because it determines the transition temperature, which indicates where it is safe to use for the required application. There are several factors affecting the DBTT curve. Crystal structure Interstitial atom Grain size Heat treatment Specimen orientation Specimen thickness

3 Effect of crystal structure Only BCC structure materials experience ductile to brittle transition temperature. be careful to select the service temperature. This is due to limited active slip systems operating at low temperature. very low plastic deformation. Increasing temperature allows more slip systems to operate more plastic deformation. FCC and HCP metals do not experience ductile to brittle transition, therefore they give the same energy absorption at any temperatures. Relationship between energy absorption and test temperature

4 Effect of interstitial atom Carbon and manganese contents have been observed to change the DBTT curve. Carbon content Smoother curve Higher Transition temp Become ductile at higher temperature Ex: in steel Mn: C ratio should be at least 3:1 to satisfy notch toughness. P, Si, Mo, O raise the transition temperature while Ni is beneficial to notch toughness. Effects of carbon content on DBTT curves for steel

5 Effect of grain size Grain size has a strong effect on transition temperature. Grain size Transition temperature Small grain size Absorbed energy, J T 3 T 3 Temperature Large grain size Reducing grain size shifts the DBTT curve to the left has a wider range of service temperatures. Heat treatments that provide grain refinement such as air cooling, recrystallisation during hot working help to lower transition temperature.

6 Effect of heat treatment Tempered martensitic structure steel produces the best combination of strength and impact toughness. Tempering temperature Energy absorption DBTT curves of different alloy steel, having tempered martensitic structure

7 Effect of specimen orientation For impact test, anisotropic properties are also observed in rolled or forged products, giving different energy absorption according to specimen orientations. Longitudinal (B) shows the best energy absorption because the crack propagation is across the fibre alignment. Transverse (C) gives the worst energy absorption because the crack propagates parallel to the rolling direction. Effect of specimen orientation on DBTT curve

8 Effect of specimen thickness Larger specimen size (in-service components) provides higher constraint more brittle. If large size specimens are used, the transition temperature will increase. Effect of section thickness on transition temperature Large scale tests

9 Drop-weight test and other large scale tests Several techniques have been developed to test specimens with different sizes to suit the applications. The specimen thickness is at least 25 mm. 1) Explosion-crack starter test 2) Drop-weight test (DWT) 3) Dynamic-tear test (DT) 4) Robertson crack-arrest test

10 Explosion crack starter test The plate was placed over a circular die and dynamically loaded with an explosive charge. The brittle weld bead introduces a small natural crack in the test plate. The test is carried out over a temperature range, giving different fracture appearance. NDT nil ductility temperature FTE Fracture transition elastic FTP Fracture transition plastic Fracture appearance vs temperature Plate dimensions : 350x350x25 mm 3

11 Drop weight test Robertson crackarrest test. Dynamic tear test

12 Embrittlement in metals Temper embrittlement Hydrogen embrittlement Stress corrosion cracking Liquid metal embrittlement Neutron embrittlement Delayed fracture curve

13 Reference Dieter, G.E., Mechanical metallurgy, 1988, SI metric edition, McGraw-Hill, ISBN