Stress Relaxation Cracking

Stress Relaxation Cracking (SRC) is arguably one of the most underrated cracking phenomena. Sudden equipment and pipeline failures at high temperatures, often occurring shortly after start-up, have typically been misattributed to overheating, creep, or other high-temperature damage modes. This chapter provides comprehensive information on SRC, including its recognition during failure analysis and strategies for its prevention.

General Information

Interest in Stress Relaxation Cracking, sometimes referred to as Reheat Cracking, Stress Relief Cracking, or Strain Oxidation Cracking, began to rise approximately 20-30 years ago. This increase coincided with a growing number of “mysterious” failures observed in austenitic stainless steels and nickel alloys operating within theoretically safe temperature ranges. Failures of welded pipelines and equipment made from high-temperature stainless steels (such as 321ss - UNS S32100) or nickel alloys (such as 800H - UNS N08810), characterized by an intergranular cracking pattern, were often attributed to modes such as creep cracking assisted by oxidation.1 2 3 Studies by van Wortel and others have confirmed that the relaxation of accumulated stress, particularly in cold-worked areas such as bends or near welds’ Heat-Affected Zones (HAZ), aided by high temperatures (typically below the maximum service temperature limits for the given steel), is the primary driver for SRC.5

Despite the research efforts made over the last two decades, several areas remain open for study. These include the impact of alloy impurities, determining SRC critical temperature, and developing a more quantitative approach to assessing and predicting SRC likelihood. The slow development of knowledge on SRC is exemplified by successive changes and updates in the corrosion mechanisms normative API RP571.

In its early release, this normative primarily recognized SRC (commonly referred to as Reheat Cracking) with limited information on influencing factors and prevention techniques, mainly through Post Weld Heat Treatment (PWHT). However, after nearly two decades, the latest release of API RP 571 acknowledges SRC as a separate damage mode (with reheat cracking now used as a secondary term) and provides more specific details on temperature impacts, detection methods, and mitigation procedures.6 However, the focus of this normative is limited to four groups of materials:

  • 1Cr-½Mo, 1¼Cr-½Mo ,
  • 2¼Cr-1Mo-V,
  • Selected 300 series: 347 (UNS S34700), 321 (UNS S32100), 304H (UNS S30409),
  • nickel-based alloys (detailing only 800H/HT - UNS N08810/N08811).

Table 1 show some of typical locations for SRC in refining and petrochemical industries.

Table 1 SRC expected areas. after 1 2 4 6 7 8

Process UnitOperation Area affected by SRC
Ethylene PlantHT operating pipelines with welds on potentially high stress loaded areas:
• cracker coils (inlet pigtails), reported on 312H.
Hydroprocessing UnitsRecycle hydrogen heater to the reactor inlet/outlet lines made of 321 and 347.
Reforming
Fluid Catalytic Cracking		Hot-wall vessels and piping operated >480°C (900°F) especially at:
• toe of nozzle-to-shell welds
• reinforcement pads
• welds on high stress loaded pipelines.
Steam Reforming (H2 production)• Reformer outlet piping (outlet pigtails, bends) – most common areas, several reported failures of 800H/HT
• Steam superheaters tubes
UtilityHP Steam pipes made of 1Cr-½Mo ,1¼Cr-½Mo operating >480°C (900°F) especially at:
• circumferential welds at high stress loaded areas

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References

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