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Lorean Hoefert
Carbon steels in service in the offshore and oil refinery industries are susceptible to a cracking mechanism known as sulfide stress cracking (SSC) or hydrogen stress cracking (HSC) when in sour service, ie when hydrogen sulfide (H2S) is present in the process fluid. Although the cracking is described as stress cracking, the main problem is the hardness of the parent metal, weld metal and heat-affected zone (HAZ).
NACE (formerly the National Association of Corrosion Engineers) has published two specifications that provide guidance on reducing the risk of in-service cracking, these also being ISO standards. The major difference between these two primary specifications is the environmental and service condition.
Both MR0175 and MR0103 have virtually identical requirements for specifying parent metal properties of carbon steels for sour service; the major concern of the standards is the requirement of a maximum hardness. All steels that have been cold-worked must be stress-relief heat-treated to ensure the hardness is less than 22HRC (Rockwell hardness, equivalent to 248Hv or 237HB). Carbon steels other than P1 can be used provided that their hardness is also less than 22HRC (237HBW). Consideration must also be given to the resultant hardness of welds.
Whilst both MR0175 and MR0103 cover a wide range of materials (carbon steels, stainless and duplex steels, nickel alloys and aluminium alloys), SP0472 is only concerned with carbon steels, classified as P1, Group 1 or 2 in ASME IX. These are hot finished carbon steels with a specified ultimate tensile strength less than 485MPa (70,000p.s.i.). Note that the BS EN 10028 steels are now assigned P numbers in ASME IX.
It should be remembered that parent metals may be weld-repaired as part of the plate production process. These base metal repairs must also comply with the NACE requirements with respect to weld metal and HAZ hardness. In addition, although SP0472 is concerned with the results of welding, any thermal cutting process will produce a heat-affected zone, which if not removed or welded over may result in HSC. In these cases, it is generally considered necessary to remove approximately 3mm of material to ensure that there are no areas of unacceptably high hardness.
Two control methods are considered for the weld deposit for prevention of cracking. Either production weld deposit hardness must be limited to 200HBW, with hardness testing of production welds to demonstrate this, or particular welding process/filler metal combinations which are specified as exempt from hardness testing must be used.
The exempt combinations are MMA welding with E60xx or E70xx electrodes, TIG welding with ER70S-2, -3, -4 and -6 filler and MAG welding with ER70S-2, -3, -4 and -6 filler. MAG welding must be performed in either the globular, spray or pulsed transfer modes. ER70S-6 filler is only exempt if it complies with compositional restrictions (C
One important aspect of the use of exempt combinations is that it may be difficult to achieve this maximum hardness figure when there is a large amount of dilution from the parent metal, for example when depositing root passes or from single-pass fillet welds where very close control of the welding parameters is required. In these cases, consideration may be given to performing some testing depending on design requirements.
SP0472 requires that the HAZs of all pressure boundary welds and internal attachment welds as well as repair welds and some external attachment welds in pressure containing equipment comply with a maximum hardness of 248Hv10.
To minimise the risk of producing unacceptably hard HAZs it is recommended to use steels with a carbon equivalent (CE) less than 0.43 (0.45 for components greater than an inch thick) where the C content of the steel is greater than 0.18wt%. Where the carbon content is less than 0.18wt%, the maximum CE shall be specified by the user. Limits are also placed on the vanadium and niobium content and consideration must be given to micro-alloying as discussed in Appendix A.
This is less of a problem with many of the BS EN steels as these are specified to have low carbon contents or a maximum CE less than 0.42. The ASME steels are permitted far higher carbon contents with no requirement to specify all of the elements required by the CE formula so care needs to be taken when ordering pressure containing materials against the ASME codes.
PWHT must be performed correctly, and so a PWHT procedure must give consideration to process, heating and cooling rates, hold times, hot zones, measurement positions and tolerances of all of these. Some guidance on the application of PWHT is given in Appendix D of SP0472.
As mentioned above, the first possible thermal method for controlling HAZ hardness is to control the cooling time of the weldment between 800C and 500C (1470F and 930F). This prevents the generation of hard microstructures.
The second possible thermal method is temper bead welding, which is a method of reducing the hardness of HAZs by using the heat input from subsequent weld runs to refine and temper the HAZ of underlying weld passes. Clause QW-290 of ASME IX specifies the requirements for temper bead welding, essential variables and weld procedure qualification.
The technique is very useful when there is a need to carry out a local weld repair but requires very precise placing of weld runs and substantial skill on the part of the welder to ensure a correct and consistent bead overlap and travel speed and that the temper bead layer does not overlap onto the base metal HAZ. A lengthy training period for the welder is likely to be required before the welder can successfully pass the qualification test and apply the technique in production.
Welding procedure qualification is the most common method of verifying that the methods put in place to control the hardness generate welds complying with the hardness requirements. It is carried out in accordance with the ASME IX requirements using actual production material or a steel of the same grade but with the maximum carbon equivalent of material to be used. The welding variables are recorded during welding of the test piece and hardness testing is mandatory, the hardness of the test weld HAZ to be less than 248Hv10, that of the weld metal less than an average of 200HBW. Hardness testing surveys are as described in NACE MR0103. In addition to the ASME IX requirements, SP0472 requires butt welds and fillet welds to be qualified separately; although not mandatory, it would also be advisable to qualify separately single and multi-pass fillet welds. The hardness in a single pass fillet weld can easily exceed 300Hv, particularly when welding on thick steel, say over 25mm thick.
Test piece thickness (and hence the cooling rate) may be an issue since ASME IX allows production components to be twice the thickness of the qualification test piece. Where PWHT is not carried out on the thicker components, thought needs to be given to whether the procedure qualification test is carried out using the thinnest test piece allowed by the code or using a test piece matching the maximum production thickness.
The welding procedure specifications (WPS) to be used in production must contain parameters matching those of the qualification test piece. Production welds must not differ more than -10% and +25% of the test piece and heat input, preheat and interpass temperatures must be the same as or greater than those of the test piece. Production welding is restricted to the same specification and grade of steel with matching or lower carbon equivalents.
Quality control must be based on best practice with well-trained and qualified welders supervised by an adequate number of competent welding foremen and inspectors. Post-weld inspection and NDE will be as required by the construction code. SP0472 does not make hardness testing of production welds a mandatory requirement but since an acceptably low hardness is crucial to satisfactory in-service performance and is sensitive to so many variables, it is advisable to perform some checking of weld and HAZ hardness on completion. This requires the use of portable hardness testing equipment and Job Knowledge articles numbers 74 and 75 discuss some of the methods available.
This paper summarizes the development and objectives of NACE International Standard Recommended Practice RP0472-95, Methods and Controls to Prevent In-Service Environmental Cracking of Carbon Steel Weldments in Corrosive Petroleum Refining Environments. Two types of environmental cracking are included; cracking due to hydrogen charging and stress corrosion cracking (SCC). Welderments are defined to include the weld deposit, base metal heat affected zones (HAZS)and adjacent base metal zones subject to residual stresses from welding.
The foreword of RP0472 contains a good history of the standard?s development, hence this section is primarily excerpts from that. Most petroleum refining equipment and piping is constructed of carbon steel having a minimum specified tensile strength of up to 485 MPa (70,000 psi), and in nearly every case, the equipment is fabricated by welding. ?The welds for refinery equipment and piping are made to conform to various codes, and according to these codes, these carbon steels are classified as P-1, Group 1or 2. Hence, in RP0472, they are referred to as P-1 steels.
Petroleum refineries as well as gas- and oil-processing plants have extensively used P-1 steels for wet hydrogen sulfide (H2S) or sour services. They are the basic materials of construction for heat exchangers, pressure vessels, storage tanks, and piping. Decades of successful service have shown them to be generally resistant to a form of hydrogen stress cracking (HSC) called sulfide stress cracking (SSC). HSC has been shown to occur in high-strength materials, or zones of a hard or high-strength microstructure in an otherwise soft material.
In the late 1960s, a number of SSC failures occurred in hard weld deposits in P-1 steel refinery equipment. The high harnesses were found to be primarily caused by submerged arc welding (SAW) with active fluxes and/or additions of alloying elements, both of which resulted in increased hardenability of the weld deposit. High harnesses were also found in gas metal arc welds with high manganese and silicon contents. To detect hard weld deposits caused by improper welding materials or procedures, the petroleum refining industry began requiring hardness testing of production weld deposits under certain conditions and applied a criterion of 200 Brinell (HB) maximum. These requirements were given in previous editions of this standard and in a similar API document discussed below.
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