Temperbead Welding

Dissimilar metal overlays and welds are commonly used in the Power Generation, Oil and Gas, and Petrochemical industries. The most common joints consist of low alloy steel and/or stainless steel components joined or overlaid with Ni-base or stainless steel filler metal. These weldments require a post-weld heat treatment (PWHT) to temper the martensite that forms in the steel heat-affected zone (HAZ), so that the hardness is reduced and the impact toughness is improved.

Several phenomena occur during PWHT that can potentially lead to failures in service: 1) carbon migration from HAZ to fusion boundary region during PWHT results in a formation of a hard and brittle band along the fusion boundary. 2) loss of mechanical properties in HAZ due to over-tempering. 3) hydrogen embrittlement in fusion boundary region, due to high hardness created by carbon migration. In addition to these potential failure issues, PWHT in the field is time consuming and often impractical.

Temper bead welding could be a viable alternative to traditional PWHT, which is difficult to perform in the field. Ultimately, the temper bead procedure will be optimized to meet specified hardness and impact toughness requirements.

 

Phase II:

Post weld heat treatment (PWHT) is a major step in the manufacturing process of matching filler metal welds (MFWs), dissimilar metal welds (DMWs) and weld overlays (WOLs) for structural and corrosion resistance applications in the oil and gas, petrochemical, and power generation sectors. PWHT is performed to restore the base metal properties in the heat affected zone (HAZ) adversely affected by the weld thermal history. Desired PWHT outcomes include reduction of hardness, improvement of toughness, restoration of creep properties in the HAZ, and relieving of welding residual stresses.


Standardization bodies as ASME, NACE, API, ASTM, ISO specify hardness and toughness requirements and/or typical PWHT conditions for various applications of structural and corrosion resistant welds and overlays. However, industry experience has shown that improper application of PWHT or even PWHT designed to fulfill particular code requirements may result in loss of properties and service failures.


A typical example of adverse effect of PWHT is carbon depletion of the coarse-grained HAZ and carbon accumulation and formation of fresh martensite in the compositional transition zone of DMWs and WOLs of carbon steels with austenitic filler metals. Carbon accumulation and hard microstructure in the transition zone of DMWs and WOLs is the main cause for hydrogen assisted cracking failures in subsea oil and gas, and in refinery installations. Carbon depletion in HAZ and carbon accumulation along the fusion boundary is related to creep and creep-fatigue failures in DMWs in fossil power plants. Exceeding the A1 temperature during PWHT in creep resistant steel welds results in formation of ferrite and fresh martensite on cooling and causes loss of creep properties and toughness.


PWHT is an expensive step in the fabrication process that can involve long dwells at high temperatures (i.e. up to 10 hours at 650 oC), may require utilization of large furnaces for large welded components, and is not readily applicable in field welding. Application of local PWHT poses technical difficulties in achieving uniform temperature profile and through-thickness properties.

 

Temperbead welding is frequently used in industry as a substitution of PWHT to fulfill hardness and toughness requirements in manufacturing of new components and in repair welding. Temperbead welding procedures are typically developed using a trial and error approach and require full procedure qualification. Changes in base material composition, component thickness, and/ or weld geometry usually require development and qualification of new temperbead welding procedures.


There is a need for creation of a scientifically based, comprehensive methodology for development of temperbead welding procedures that accounts for materials response to hardening and tempering in the conditions of multipass welding and for the effect of welding parameters and weld geometry. Industrial implementation of such methodology can result in significant reduction of production cost and in improved service performance of MFWs, DMWs, and WOLs in oil and gas, subsea, petrochemical, and power generation installations.

 

Industry Sponsor: EPRI

Faculty: Boian Alexandrov (OSU)

Graduate Students: Eun Jang, Yuxiang Luo, Tom Nemcek

Industry Contact: Steve McCracken, Darren Barborak