To Balance Hardness and Cracking by Adjusting Welding Parameters
Hardfacing alloys are commonly used to enhance the lifecycle of substrates against to mostly abrasive wear and corrosion. Cobalt, nickel and iron based alloys offer diverse attributes to the welding deposit. A general idea of increasing the service life in terms of wear is to make the surface hard, which is obtained by the carbon element in the matrix which forms hard carbide. This can be evidenced by typical Stellite alloys in the following table:
| C% by weight | Hardness |
|
|---|---|---|
| Grade 190 |
3.3 |
52 - 60 |
| Grade 1 | 2.0 - 3.0 |
50 - 58 |
| Grade 12 | 1.4 - 1.7 | 45 - 51 |
| Grade 6 |
0.9 - 1.4 | 36 - 45 |
It's easily found that the hardness gets bigger with the increased carbon ratio.
A drawback of getting hard is that the harder a matter is, the more easily crack occurs. Even if the crack is not significantly considered as the important factor that reduces the service life due to its impact on lowering residual stresses, cracking is unexpected by most applications e.g. hardfacing, sealing surfacing and fatigue resistance.
Below are snippets from catalogue of Kennametal STELLITE in which it can be found clues that cracking is paid attention to:
- Grade 190: "highly abrasion-resistant alloy", "avoid severe mechanical or thermal shock"
- Grade 1: "crack-sensitive", "minimize cooling stresses"
- Grade 12: "intermediate alloy between 1 and 6", "often used self-mated or running against 1 or 6"
- Grade 6: "good all-round performance", "industry standard for general wear resistance purpose"
There are two kinds of crack, one is during solidification and another is in the solid status. The former can be solved by reducing the fluidity of the steel water while the latter is because of low tensile ductility. Tensile-ductility-related crack is essentially caused in the hardfacing process in which material composition, microstructure and welding condition play important roles.
To govern the risk of cracking, there are two crucial aspects of welding condition that include preheat temperature and deposit dilution. The dilution levels can be changed by the adjustment of the current of either TIG, MIG, PTA or MMA. As lower dilution and higher deposit hardness are pursued more often in hardfacing applications, Wisdom has been experiencing abundant cases of optimizing the parameters of welding process, mastering the sensitivity to crack. Moreover, we are now conducting welding qualifications of various base metals for better servicing our customers.
