NACE - SP0472
Methods and Controls to Prevent In-Service Environmental Cracking of Carbon Steel Weldments in Corrosive Petroleum Refining Environments
|Publication Date:||14 March 2015|
This standard establishes guidelines to prevent most forms of environmental cracking of weldments in carbon steel refinery equipment, including pressure vessels, heat exchangers, piping, valve bodies, and pump and compressor cases. Weldments are defined to include the weld deposit, base metal HAZ, and adjacent base metal zones subject to residual stresses from welding.
Complete PWHT of field-fabricated storage tanks is impractical and sometimes impossible. Costly techniques of supporting tanks, insulating the outside, and applying gas burners to heat the inside gas have been attempted, but the effectiveness was questionable. Other protection steps such as alternative thermal methods, internal coatings, maintaining lower temperatures, etc. should be used.
This standard covers only carbon steels classified as P-No. 1, Group 1 or 2. These classifications can be found in the ASME Boiler and Pressure Vessel Code, Section IX9 for pressure vessels, ASME/ANSI B31.3 for process piping, or API Standards 620 and 650 for tanks. It excludes steels with greater than 485 MPa (70,000 psi) minimum specified tensile strength. Other materials may be vulnerable to cracking, but these materials are outside the scope of this standard.
The types of equipment covered by this standard include pressure vessels, heat exchangers, piping, valve bodies, and pump and compressor cases. All pressure-containing weldments or internal attachment weldments to the pressure boundary are included. External attachment weldments are sometimes included as discussed in Paragraph 3.5.1. In addition, this standard may be applied to weldments in some non-pressure-contain
Both new fabrication and repair welds are within the scope of this standard. The practices included herein are intended to prevent in-service cracking and are not intended to address cracking that can occur during fabrication, such as delayed hydrogen cracking. In most cases, however, these practices are also helpful in minimizing these fabrication problems. Useful information for preventing delayed hydrogen cracking is provided by F.R. Coe, et al.10
Welding processes covered by this standard include shielded metal arc welding (SMAW); gas metal arc welding (GMAW); flux-cored arc welding (FCAW); gas tungsten arc welding (GTAW); and submerged arc welding (SAW). Almost all types of weld configurations are included. For specific exceptions, such as hot taps, hardness limits and post weld heat treatment (PWHT) requirements should be reviewed on a case-by-case basis.
Corrosive refinery process environments covered by this standard can be divided into two general categories: services that could cause cracking as a result of hydrogen charging, and services that could cause ASCC. However, identification of the specific environments to which the guidelines set forth in this standard are to be applied to prevent various forms of in-service environmental cracking is the responsibility of the user. Figure 1 is a simplified schematic showing the interrelationships of the various cracking mechanisms discussed in this standard.
Services that could cause cracking as a result of hydrogen charging:
In these services, the environment or corrosion reactions result in diffusion of atomic hydrogen into the base metal and weldment. In high-strength or high-hardness areas, this hydrogen can result in HSC. In petroleum refining processes, the primary manifestation of HSC is SSC of hard weldments in process environments containing wet H2S. Environmental conditions known to cause SSC in carbon steels are discussed in NACE Standard MR0103. However, other processes that promote aqueous corrosion of steel and promote hydrogen charging (such as hydrofluoric acid) can also cause HSC. Controlling both the weld deposit and HAZ hardness using the guidelines in Section 2 prevents HSC in most cases.
SOHIC can also occur in the services described above, but it does not require high strengths or high hardnesses. Hence, limiting weldment hardness does not prevent this form of cracking. Reducing weldment hardness and residual stress is believed to reduce the likelihood of this cracking, so the guidelines in Sections 2 and 3 may still be helpful. However, additional steps, such as the use of special clean steels, water washing, corrosion inhibitors, or corrosion-resistant liners, may be needed for some services. An overview of the materials selection, fabrication, PWHT, and testing practices that have been applied to new pressure vessels for preventing SOHIC is in NACE Publication 8X194.12
Cases of cracking of hard welds have occurred as a result of short-term upset, start-up, or transient conditions in non-stress-relieved P-No. 1 steel refinery equipment in which hydrogen sulfide is not normally present.
Although this standard covers only P-No. 1 steels, welds have also cracked in tanks and pressure vessels constructed of non-stress-relieved P-No. 10A and 10C carbon-manganese steels.
Services that could cause ASCC:
Figure 1 provides examples of environments that could cause ASCC, including caustic stress corrosion cracking, amine stress corrosion cracking, and alkaline carbonate cracking (commonly referred to as carbonate cracking). Section 3 provides common practices used to prevent these types of ASCC. Severity of cracking is often dependent on temperature, concentration, level of residual tensile stresses, and other factors. Controlling weldment hardness does not prevent ASCC because high tensile stresses still may be present.
Further information about caustic cracking and its prevention is in NACE SP0403.13
Further information about amine cracking and its prevention is in API RP 945.14
Further information about carbonate cracking and its prevention is in NACE Publication 34108.15
It is outside the scope of this standard to detail all the specific environments causing ASCC of P- No. 1 steels. Various reference books and publications contain information on ASCC environments and preventive measures.13-16
One possible environmentally induced cracking mechanism in carbon steel weldments that is not addressed in this standard is high-temperature hydrogen attack. API RP 94117 gives recommendations on materials selection to avoid this problem. Other types of in-service cracking not addressed by this standard are primarily mechanical in nature (e.g., fatigue, creep, and brittle fracture).
This standard was reorganized in 2008 to present the standard practices in a specification format in the main body. All other supporting information and guidance are now in appendixes.
Appendix A (nonmandatory) provides the rationale for the guidelines in Section 2 for prevention of HSC. The paragraphs in Appendix A are numbered to correspond with the related paragraph in the main body of the standard for which it is providing the rationale (e.g., Paragraph A.2.3.2 in Appendix A corresponds to Paragraph 2.3.2 in Section 2).
Appendix B (nonmandatory) provides the rationale for the guidelines in Section 3 for prevention of ASCC. The paragraphs in Appendix B are numbered to correspond with the related paragraph in the main body of the standard for which it is providing the rationale.
Appendix C (nonmandatory) provides a summary of the cooling time (t8/5) concept discussed in Paragraph 184.108.40.206.
Appendix D (nonmandatory) provides definitions for local PWHT terminology used in Paragraph 3.7.
Table 1 provides an overview ("road map") of the guidelines applicable to the various types of cracking.