African Fusion June 2019

Cold repair of 625 HSS

This paper, presented at the IIW Annual Assembly Commission 2 in Indonesia (Bali) in 2018, won its principle author, Kristian Kruger, the 2018 Harvey Shacklock Gold Medal Award for the best technical paper presented at an SAIWevent. The paper was co-authored by Frederic Laurent and Pieter Pistorius. Cold repair of Inconel 625 clad layers on medium carbon high strength steels KJ Kruger, F Laurent and PGH Pistorius

M edium carbon steel ASTM A519 Gr 4130 pipes and forgings are often clad with Inconel 625 before being placed in service in the subsea oil and gas industry. Due to the service conditions, itmay be necessary to repair the Inconel cladding without subjecting the underlying alloy to a second post weld heat treatment (PWHT) cycle. A ‘cold repair’ technique was developed with the intention of removing cladding such that a standard amount of the clad thickness remains, followed by a weld build up. The new build up requires low heat input such that a new HAZ does not form in the medium carbon steel and PWHT is not neces- sary to temper the heat affected zone. The effect of changes in welding parameters, specifically the welding speed, on the dimensions of the repair weld bead was estimated using the thick-plate Rosenthal equation. The study showed that it is possible to repair weld remain- ing Inconel cladding with a minimum 1.5 mm clad thickness using pulsed gas metal arc welding (GMAW-P). The results of the study showed no visible new HAZ in the medium carbon steel, no increase in medium carbon steel HAZ hardness, as well as sufficient fusion at the Inconel-Inconel interface. 276 MPa 30% Table 2: Minimum tensile properties of solution-annealed Inconel 625 plate as per SB-443 Gr 2. Introduction Inconel 625 is a solution strengthenednickel-basedalloy that is well known for its high tensile strength, impact toughness and corrosion resistance in chloride and acid containing environ- ments at a range of temperatures. Table 1 and Table 2 show the typical chemical composition andmechanical properties of In- conel 625plate [1]. The combinationof corrosion resistanceand mechanical properties makes Inconel 625 an ideal candidate formarine applications. Inconel 625 is often used as a cladding material to impart these beneficial properties to the surface of other materials in order to reduce the cost of fabrication. Initial fabrication During fabrication of components intended for use in the ma- rine industry, several factorsmust come into balance, namely: material requirements such as tensile strength, corrosion 690 MPa %Cr %Mo %Nb+Ta %Fe 8-10 3.15-4.15 <5.0 %Co <1.0 %S 20-23 0.015 %P %C 0.1 %Al %Si %Mn %Ni 0.015 Bal Table 1: Chemical composition of Inconel 625 plate as per SB-443 [1]. 0.4 0.5 0.5 Yield strength Tensile strength Elongation

resistance, hardness, and impact toughness; fabrication lead time or availability; and cost. While inmany cases Inconel 625 would conform to or exceed design requirements, in general the feasibility in terms of lead time and cost to produce large complicated components out of the alloy makes this option entirely unfeasible. For this reason, Inconel 625 (ERNiCrMo-3) cladding is often applied, typically to a layer thickness rang- ing from 3.0 to 5.0 mm onto a selected surface of a more affordable material such as ASTM A519 Grade 4130. Table 3 and Table 4 show the chemical composition and mechanical properties of 4130. Themost common claddingmethod is through arcwelding followed by a post weld heat treatment (PWHT) of the clad components.

%C

%Cr

%Mo

%Mn

0.28-0.33

0.9-1.1

0.15-0.25

0.4-0.6

%Si

%S

%P

%Fe

0.10-0.35

0.04

0.035

Bal

Table 3: Chemical composition of ASTM A519 Gr 4130 [1].

Yield Strength Tensile Strength

585 MPa 725 MPa

Elongation

10 %

Hardness (HRB) 95 Table 4: Typical mechanical properties of ASTM A519 Gr 4130 in the stress relieved condition [2]. Repair of worn cladding During service of larger moving parts that have been clad, the Inconel cladding experiences degradation that affects the performance of the component. In these cases, it is necessary either to repair the cladding or to scrap the component. Scrapping and replacing large components is extremely costly and not the preferred route. Should the component undergo conventional repair, the route would be to remove and re-apply the cladding. This would be followed by a PWHT identical to that specified in the initial cladding application. However, it is often the case that if the steel is exposed to an additional welding cycle and subsequent PWHT, the base metal mechanical properties could fall below theminimumre- quirements for the component. In order to avoid this scenario, a technique that does not expose the medium carbon steel to a weld thermal cycle that results in austenitisation must be employed to repair the damaged cladding and to obviate the need to perform a PWHT. This technique is referred to as a cold repair and this paper sets out the results of such a cold repair on ASTMA519 Gr 4130 clad with Inconel 625.

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June 2019

AFRICAN FUSION