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  • FAQ: Why do the proof and tensile strength of Ni-based weld metal (used for welding 9% Ni steels) differ in welds deposited in different positions?

Why do the proof and tensile strength of Ni-based weld metal (used for welding 9% Ni steels) differ in welds deposited in d

   

Frequently Asked Questions

Ni-based weld metal (used for welding 9% Ni steels) has a strong tendency to exhibit aligned grains, which means that the slip planes are aligned and so there may be greater or lesser tendency for slip depending on the orientation of the load bearing direction in the weld.

Several investigators[1-5] have found that longitudinal specimens taken from weld metal deposited in the vertical-up position shows a higher strength than that deposited in the horizontal-vertical or downhand positions. The reasons for this are not completely understood, and the varying welding conditions and metallurgical texture have been implicated in why this occurs.

Blake, Rowntree and Phelps[1] found that the grains from the vertical-up weld had their axes approximately 45° to the longitudinal axis of the weld and approximately 90° to the transverse. They also found that the horizontal-vertical weld had grain axes approximately 90° to the longitudinal weld axis and approximately 45° to the transverse. The downhand weld had grain axes perpendicular to both the longitudinal and transverse axes of the weld.

Welland[3] found that clusters of {200} poles corresponded with the growth direction. For the vertical up weld, this was 15° towards the welding direction; for the horizontal-vertical weld it was 30o transverse to the welding direction and for the downhand weld, the axes were clustered near the welding plane normal. These differences resulted in the same trend in results, where the downhand specimens exhibited little difference in longitudinal and transverse weld properties, the horizontal vertical specimens exhibited higher strength in the transverse weld direction and the vertical-up specimens exhibited higher strength in the longitudinal weld direction.

More recent work by Gustafsson et al[4] identified that heat input affects texture and that using higher heat inputs in horizontal welds results in some angling of the preferred growth direction towards the welding direction, such that although vertical-up welds were strongest, and horizontal welds weakest, high heat input (>2kJ/mm) horizontal welds had higher tensile strength than low to medium (0.4-1.1 kJ/mm) heat input welds.

Miller[5] has also carried out testing, considering the effects of welding parameters on dilution and thereby the effect on strength. This work also identified preferential grain directions in the weld metal, but concludes that strength can also be linked to dilution. The greater the dilution (welding current), the lower the strength.

  1. BlakeM A W, Rowntree G and Phelps B: 'The variation in strength of a weld metal for 9% Ni steel', Preprints, Conference on Welding Low Temperature Containment Plant, London, 20-22 November 1973, Paper No. 1, pp1-9.
  2. Hanova E: 'MMA welding of 17 501 (9% Ni) steel', Welding International, Vol. 2, No. 8, 1988, pp 755-762.
  3. WellandW G: 'Producing strong welds in 9% Nickel steel', Welding Journal, Vol. 57, No. 9, September 1978, pp 263s-271s.
  4. GustafssonM, Thuvander M, Bergqvist E L, Keehan E & Karlsson L: 'Effect of welding procedure on texture and strength of nickel-based weld metal'. Science and Technology of Welding & Joining, Volume 12,Number 6, August 2007, pp.549-555
  5. MillerC J: and Nicholas J M: 'Relationship between Ni-base weld metal strength, welding current & weld metal dilution in 9% Ni steel joints' TWI Research Report 950/2010.

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