Introduction
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.
The first standard, NACE MR0175/ISO 15156, ‘Petroleum, petrochemical and natural gas industries – Materials for use in H2S-containing environments in oil and gas production’, is intended for offshore applications. The second standard, NACE MR0103/ISO 17945, ‘Petroleum, petrochemical and natural gas industries – Metallic materials resistant to sulfide stress cracking in corrosive petroleum refining environments’, is intended for onshore process plant.
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.
Standard practice guidance
MR0103 refers to a standard practice document for controlling welding activities, NACE SP0472, ‘Methods and controls to prevent in-service environmental cracking of carbon steel weldments in corrosive petroleum refining environments’. SP0472 and this article are concerned with the methods used to control the weldment hardness to prevent SSC and HSC. SP0472 also gives some consideration to prevention of alkaline stress corrosion cracking (ASCC) via post-weld heat treatment (PWHT) but this will not be covered in this article.
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.
Welding processes covered by SP0472 are the more common processes; manual metal arc (MMA), metal active gas (MAG), flux-cored arc welding (FCAW), tungsten inert gas (TIG) and submerged arc welding.
SP0472 provides a ‘Road Map’ of guidelines, with the aim of preventing the two cracking mechanisms by controlling weldment hardness. There are three major components to this. The first is prevention of the HSC or SSC cracking mechanism by control of the weld deposit. The second is prevention of the HSC or SSC cracking mechanism by control of the HAZ. The third is prevention of the ASCC cracking mechanism by PWHT of the entire weldment.
Control of weld deposit
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<0.1 wt%, Mn<1.6 wt%, Si<1.0 wt%) which must be confirmed by compositional analysis of the filler, performed either by the supplier or the user. This implies that additional filler metal certification and batch control on the shop floor is necessary to demonstrate compliance.
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.
Control of heat-affected zone
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.
The SP0472 road map provides two overarching approaches to achieving this HAZ hardness. Both approaches require control of the chemistry of the base metal and then an additional control method. The first additional control is to apply PWHT to the weldment. The second possible additional control is to apply one of the ‘thermal methods’ suggested by SP0472, namely cooling time control or temper bead welding, and then perform a HAZ hardness survey during the procedure qualification to verify the thermal method was successful.
Base metal chemistry control
The base metal chemistry control is concerned with the carbon equivalent (CE) of the metal, calculated by the formula:
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.
Additional control 1 – Post weld heat treatment
PWHT tempering will reduce both the hardness of a weld and the residual stresses and both of these factors will reduce the risk of cracking. Depending on the construction code there may, in any case, be a requirement to PWHT – ASME VIII, Unfired pressure vessels, requires PWHT over some 32mm thick, ASME B31.3, process piping, when thickness exceeds 19mm. As high a PWHT temperature as possible should be used to achieve the maximum amount of tempering. BS EN 13445 Part 4 – the pressure vessel code – permits PWHT temperatures as low as 550°C and there is also an option in BS PD 5500 to use a similar low temperature. Such low temperatures may not give the required reduction in hardness and consideration should be given to prior experience.
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.
Additional control 2 – Thermal methods
As mentioned above, the first possible thermal method for controlling HAZ hardness is to control the cooling time of the weldment between 800°C and 500°C (1470°F and 930°F). This prevents the generation of hard microstructures.
The minimum cooling time ‘t8/5’ or cooling rate for production welding must be specified. The calculation of this cooling rate is described in Appendix C of SP0472 and takes into account the joint configuration, preheat and the process heat input. The method is qualified by carrying out a pre-production weld test on representative parent material using the fastest cooling rate at which the HAZ hardness is acceptable; several tests may therefore be required. A successful test potentially qualifies all other production welds made with cooling rates slower than that of the test piece, calculated from the formulae in Appendix C. This may require the welders to be specifically trained to deposit weld metal within very tight limits on travel speed, weaving etc. and will require close supervision during production welding possibly with temperature monitoring.
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.
Preproduction hardness survey
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 revision by Rob Shaw is based on an earlier article by Gene Mathers, updated in line with the current version of the relevant standards.)