Challenges in Welding High Strength Steels and High Strength Corrosion Resistant Alloys
Authors: Badrak Robert P. , Howie William , Scott Joe Lynn 
Published: May 2011

The application requirements including various combinations of corrosion resistance, high strength, high impact/fracture toughness, and environmental cracking resistance are presented in relation to the challenges that they represent for welding. Achieving these requirements is discussed in relation to welding process limitations, material limitations and availability of suitable consumables. Several examples are presented that illustrate the problems associated with meeting design requirements.

Introduction

Welding is performed for a variety of purposes such as (1) meeting design requirements, (2) economic reasons and/or (3) local preferences. The most common reason is that the structure geometry lends itself more to weld fabrication than other competing processes such as casting, forging and machining. In meeting design requirements, often combinations of properties are required or for sealing integrity that necessitate weld joining. For pressure containment or isolation, sealing through weld fabrication has greater reliability and capability compared with mechanical means of sealing. Welding fabrication is an optional (or economic) approach to manufacturing but often becomes a necessity because of geometry.

The use of weld fabrication in the Oil & Gas Industry is very widespread with oilfield examples including pipelines, wellhead equipment, rig structures, pipe handling equipment, gas lift equipment, sand screens, plunger lift equipment, coiled tubing, drill through equipment and compressors. The reasons are as varied as the equipment listed above. For example, pipelines are welded primarily for integrity reasons where leaks are rarely acceptable. Wellhead and Drill-Through equipment designs generally require extensive weld fabrication and can be avoided only with a huge cost penalty. Gas lift equipment cannot be reasonably manufactured without using weldments because of the geometric limitations imposed by valves in oval tubing structures. Pipe handling equipment and subsea valves and devices have geometric constraints and design requirements that are difficult to meet without welding. In addition, the subsea products usually require welding for integrity and reliability issues.

The varied equipment with their diverse functions imposes equally varied design requirements on the completed welded form. When these design requirements are translated to include weldments we discover limitations that result from the welds' cast structure such as the fatigue toughness lower than the basemetal etc. For mechanical properties we rely on a combination of variables including consumable composition, flux and shield gas composition, heat input and heating/cooling rates to control the properties of the weld joint.

The product design requirements are further augmented by the application variables such as operating temperature ranges and harsh operating environments. The design temperature could range from −50°C (−58°F) for artic drilling operations to 350°C (662°F) for geothermal production applications. The harsh environments include those such as high concentrations of acid gases (H2S and CO2) coupled with high chlorides in production environments, marine and subsea operating environments and aerobic chloride containing injection applications. The realities of these variables add additional weld joint requirements such as corrosion resistance and resistance to environmentally induced or accelerated cracking mechanisms.

The end result of these design and application requirements are ranges or minimum values of the weld joint properties that are summarized in Table 1.