1. Cold Cracking and Segregation in Multi-Pass Welds of a Quenched and Tempered Steel
GLF Powell, IH Brown, VM Linton; University of Adelaide
JL Davidson; Defence Science and Technology Organisation

2. Hydrogen Assisted Cold Cracking of Welds
Contributing factors
Hydrogen
Stress: applied or residual
Susceptible microstructure
Susceptibility expressed in terms of carbon equivalent calculated from nominal weld metal composition

3. Back scattered electron image of a crack found in a sample welded under low heat conditions viewed in an unetched condition. The crack was a centreline cold crack.

4. Segregation in Welds Macrosegregation Microsegretation Solute banding
Normal to solidification growth direction
Microsegretation
Intercellular-dendritic
Parallel to the solidification growth direction

5. LIQUID SOLID START FINISH Macro segregation
Micro segregation of Ni, Mn, Mo, Cr
START
FINISH
Macro segregation

6. Crack path etched in Nital and crack does not appear to follow any of the lighter ferrite veining or oxide particles- indicators of the former austenite grain boundaries.

7. Etched in Le Pera’s reagent which outlines the differences in alloy composition rather than the phases. It can be seen quite clearly that the crack follows the segregation at the intercellular-dendritic boundaries.

8. Increase in microhardness HV
Increase in microhardness HV at the intercellular dendritic boundaries as compared with that within the cells. Again indicating an increasing in alloying element content in these regions and a greater susceptibility to cracking.

9. Microprobe X-ray map of manganese in the weld metal in the region of the crack. The crack is also shown but is clearer in the following slide which has been taken at higher magnification.
50x

10. 250x

11. X-ray line scan ahead of the crack showing elemental peaks at approximately 70mm. Signs of other peaks at 0mm and again at 135mm. Note the peak at for Mn (the most obvious element because it is there in the largest amount 70mm is approximately 1.4x higher than the background.
Because the structure is not purely cellular the height of the peaks varies as will the distance between them. All of the experimental work to date suggests that the ratio of the peak height to the nominal composition (background height) will be 1.4:1 and that the peaks will be between 30 and 80 microns apart.

12. Summary
Micro-segregation appears to occur on the cellular dendritic boundaries
Micro-segregation of all elements appears to be in the same ratio of 1.4:1
The micro-segregated region is harder than the matrix by the order of 100Hv
The crack path is through the intercellular dendritic micro-segregated harder regions

13. Experimental Welds Quenched and tempered steel
Multipass submerged arc welds
Weld Metal
Composition

14. Welding Conditions Carbon equivalent Ceq = 0.63 Heat input = 1.3kJ/mm
Preheat 40oC
Radiography revealed short cracks, later identified as fish eyes

15. Armed with these results we approached Len Davidson from DSTO, a researcher who has had an interest in HACC for more than 20 years and asked him whether he had any samples that had been used for HACC testing. He provided one complete compact tensile test sample (fractured) and a fractured half of another similar sample. The samples had been manufactured using SAW (63 passes) and the all weld metal samples cut from the region as shown.
The specimens machined from the weld metal had a slot machined in them. They were then radiographed and magnetic particle tested. These on-destructive tests showed that there were cracks present in the sample in the as machined condition. The samples were fatigue cracked prior to fatigue testing by DSTO.

16. Optical macrograph of the mating fracture surfaces of the Compact Tension Specimen after mechanical testing. The long arrow indicates the fish eye and the shorter arrows the curved delineation between the fracture regions.

17. FESEM of the fish eye arrowed in Figure above
FESEM of the fish eye arrowed in Figure above. The smooth initiation region containing the gas pore and the directional cleavage type fracture within the fish eye can be seen.

18. FESEM image of the gas pore shown in the earlier figure at higher magnification. The pore is shiny and the surface smooth indicating that it formed during solidification.

19. The appearance of the fracture surface at the base of the “fisheye”
The appearance of the fracture surface at the base of the “fisheye”. Note the ‘Tight” cleavage fracture and the fact that the distance between the cleaves is approximately 50 microns which is within the range of for the cell diameters in welds. This image is similar to that previously published by Davidson in the Proceedings of the Conference on Hydrogen Management in weld metals held in Melbourne in His SEM micrograph is shown in the next slide.

20. Davidson was using this as an example when discussing the correlation between toughness and resistance to hydrogen induced cracking as both are dependent on microstructure. Macro hardness and the type of consumable were generally found to be indicators of susceptibility, but not in all cases. For example higher hardness generally higher susceptibility and Mmaw generally higher susceptibility than flux cored.
From Davidson 1996

21. Optical micrograph of the cracking through a fish eye
Optical micrograph of the cracking through a fish eye. The fish eye was approximately 5mm below the fracture surface. The white marker indicates the position of the X-ray line scan shown in the following slide. Sample is unetched.

22. X-ray line scans along the line indicated
. X-ray line scans along the line indicated.. Vertical lines have been added through the peaks. Two things should be noted
- the peaks are in the range of microns the intercellular spacing predicted by Savage
- the ratio of the peak values to the background curves for the elements is approximately 1.4:1

23. X-ray maps taken through the crack shown 2 slides earlier
X-ray maps taken through the crack shown 2 slides earlier. The elements with the higher weight percents show the outlines of the cellular dendrites and their higher intensities at the boundaries of these cellular dendrites are coincident as shown for example by the arrowed boundaries.

24. Summary - Microsegregation
There appears to be a hydrogen bubble associated with the initiation point of each “fish-eye”
Micro-segregation of all elements appears to be in the same ratio of 1.4:1
The crack path is through the intercellular dendritic micro-segregated harder regions
The value of the carbon equivalent as calculated by usual methods should be increased by a factor of 1.4 to take account of the micro-segregation

25. This is an optical micrograph (unetched) of a long curved crack found on the same surface as the fish eye. This crack was unrelated to the cracking associated with the fish eye.

26. After etching in Nital it can be seen that the cracking has occurred in a banded region which appears from the shape of the austenite grains to be at the top or the bottom of a weld pass. The dark etched banding is more obvious in the following slide taken at a higher magnification near the tip of the crack.

27. End of the crack. The crack is contained within a dark etching band
End of the crack. The crack is contained within a dark etching band. Etchant 2% Nital. The band appears to be bainite/martensite. In an effort to further understand what was occurring in this region X-ray line scans and X-ray maps of the region were taken using an electron probe. The position of the line scans is shown in the next slide.

28. Repolished surface. The regions between the two sets of dark parallel lines indicate the positions of the X-ray line scans.

29. Note that manganese is the only element to show an increase in concentration. For the microsegragation shown earlier all of the elements increased in concentration by a factor of 1.4 : 1, but that is not the case for this macro-segregated region. The X-ray maps of a region across the crack tips are shown in the next slide.

30. Mn Ni Mo
X-ray maps showing the strong manganese banding but none of the other elements have segregated. Cr

31. Summary - Macro-segregation
Only manganese was found segregated in a banded region
The manganese concentration in the band was double that of the material nominal composition
Cracks were found in this macro-segregated band
The band appeared to be associated with either the top or the bottom of a weld pass
The shape of the concentration profile as revealed by X-ray analysis is the same as that predicted for a perturbation in the solidification rate

32. After Davies and Garland

33. Conclusions
Segregation, both micro and macro appears to influence the susceptibility of a weld metal to hydrogen cracking
Segregation factors could be included when using ‘rule of thumb’ measures to determine weld metal susceptibility and welding parameters