Tasman Pulp and Paper operate several large wood pulp digestors in New Zealand. Over time, these had become corroded and required renovation. TWI was called upon to give expert advice as to how to do this.
One suggestion for repair made by TWI was 'wall papering' to span the damaged region with thin stainless steel sheet, with the exposed HAZs peened to stress relieve them. However, the brief given to our contact was for a weld-clad repair. This presented the problem of potential caustic cracking (a type of stress corrosion cracking) in the HAZ at the end of the clad region.
Seeking a means of giving the exposed HAZs a stress relief heat treatment, but not wishing to subject the whole of the repaired region to an elevated temperature, TWI suggested a novel approach, which involved the use of Inconel 625 (Ni-base alloy) weld cladding. The Inconel has a thermal expansion coefficient similar to that of C-Mn steel, and it is unaffected by causing solutions such as the liquor in the tank. It was therefore well suited to the conditions, but too expensive to use for the whole repair.
The solution adopted was therefore to deposit a narrow band of Inconel 625 weld metal around the inside of the vessel, at each extremity of the region to be clad. After appropriate inspection, this band was given thermal stress relief heat treatment. The weld cladding was then deposited, with an austenitic stainless steel consumable with the ends impinging on the Inconel 625 weld metal. The resulting as-welded HAZs at each end of the clad layer were thus in Inconel weld metal, and the possibility of caustic corrosion cracking eliminated.
As part of the justification for the cladding procedure, the New Zealand certifying authorities required that the operators demonstrated fitness for purpose using fracture mechanics. The assessment involved assuming certain defect sizes and orientations, up to one bead in depth, which could have been missed by NDT. Almost all of the steel of the vessel wall was Al-treated, and therefore strain ageing was not expected to be a problem. However, one or two sheets were suspected to be not Al-treated, and therefore potentially susceptible to strain age embrittlement. Part of the project involved copying the original steel composition with a special heat of steel, and rolling to the thickness of the vessel wall. The effect of potential defects in the vessel wall was modelled using specialist fracture toughness test techniques.
The results of the combined test programme and fitness for purpose study enabled Tasman Pulp and Paper to satisfy the certifying authorities that the repaired digestor was fit for purpose, and the renovated pulp digestor is now operating safely.
For more information, please contact us.