Working with specialised steels

Paul Boughton

Ingo Detemple & Jörg Maffert reveal their experience with welding and stress-relieving heat treatment of CrMoV steels

Chromium-molybdenum steels are used for the construction of apparatus and equipment in which complex processes take place. These generally take the form of petrochemicals plants and refineries, also referred to as ‘downstream’, where chemical products and/or synthetic fuels are produced from natural gas or crude oil.
Two of these process operations are ‘hydrotreating’ and ‘hydrocracking’. Hydrocracking is a catalytic cracking process conducted in the presence of hydrogen, in order to convert higher molecular-weight hydrocarbon fractions to intermediate products for the production of motor gasoline, kerosene and diesel fuel. The process is operated at hydrogen pressures of up to 200 bar and temperatures of up to 480°C. The apparatus in which these processes take place are referred to as ‘reactors’ and are generally designed for 40 years of productive operation.

What makes these steels so special? In addition to their resistance to high-pressure hydrogen and their mechanical and technological properties, they are also required to withstand a further damage mechanism: temper embrittlement. The steel therefore needs low levels of tramp elements such as tin (Sn), antimony (Sb) and arsenic (As), targets that can be achieved using the oxygen-blowing steelmaking process. 

These steels are alloyed using Cr, Mo and, in some cases, with V, which fix the carbon contained in the steel in special carbides, i.e., chemical compounds between the alloying elements and carbon. Any hydrogen that enters the steel is fixed, at non-critical concentrations, to the finely dispersed vanadium carbides, and is thus not available for any reaction of carbon with hydrogen to form methane, a reaction that can seriously damage the steel.

Welding also requires special properties in the parent steel material. Stress relief of the structure is necessary as well as the precipitation of carbon-fixing special carbides in the weld metal. The completed vessels are annealed under extreme conditions for this purpose. The parent material is, in fact, required to withstand these heat-treating conditions multiple times and nonetheless remain able to bear its operating loads safely after completion of fabrication.

These production and welding requirements, and also subsequent heat-treating requirements, usually originate from the API (American Petroleum Institute) 934 series of documents. These documents are extremely high-ranking recommendations to industry, and are used around the world as the basis for the definition of the technical requirements.

In this paper, the authors examine the necessity and influence of post-weld stress-relieving annealing (PWHT = Post Weld Heat Treatment) in accordance with API RP 934 A, and also of heat input, on the welded joint. Requirements concerning PWHT originate from weld filler material manufacturers, material users and equipment operators. The presence of the necessary microstructure is the precondition for attainment of the necessary toughness in the weld metal. PWHT thus serves not only to reduce welding residual stresses but also, and in particular, for the achievement of the desired mechanical properties in all the possible operating states that the reactor can undergo during its service-life. This requirement must be harmonised with the parent material, the mechanical and technological properties of which are significantly impaired by excessive heat treatment. The end customer, equipment fabricator, weld filler material manufacturer and steel producer cooperate closely in agreeing PWHT conditions.

Typical conditions with reference to API RP 934 A1 and API TR 934 B2 are:

  • API RP 934 A, 3.1.6, 7.6.1 (May 2008):
  • SA 387-22-23: 690+/-14° C, holding time in accordance with ASME BPVC4 
  • SA 542-D-4a5: 705+/-14° C, holding time not less than 8h
  • Not less than three cycles in each case

Multiple cycles result from the necessity of repair welding; of these, two cycles are available to production, and one to two to the operator.

There are diverse reasons for PWHT; the aim is tempering for the achievement of the necessary microstructure in the weld metal, the HAZ and the parent material. Another aim is that of reducing welding residual stresses.

The API 934 series draws attention to the dangers that excessively ‘stringent’ heat treatment may cause. Mechanical properties may then drop below the minimum requirements.

Influence of PWHT conditions on the metal’s toughness
A single-V butt weld (without full penetration) 
was welded.

The welding-test parameters are summarised below:

  • Grade of steel: SA-542-D4a (21/4Cr1Mo1/4V)
  • Plate thickness: 105mm
  • Tempering conditions: 720°C/140 min.
  • Intermediate Stress Relief (ISR): 660°C/180 min.
  • Heat input: 18 to 24kJ/cm in every case 
  • Welding process: 121 (single-electrode SAW process) 
  • PWHT: One cycle in each case (HP value): 20.34 / 20.51 / 20.64 

Testing was performed only on the weld metal/in the weld seam, using the tensile test (transverse, at room temperature) and the Charpy V-notch impact test (four samples across weld thickness, transverse, at -29° C).

Properties of CrMoV steels as a function of HP value

The critical variable for the weld metal is toughness in PWHT min. state, i.e. the condition of the reactor during commissioning. Two influencing factors, heat input and PWHT, were examined during the test. The fabricators of pressure vessels generally use a specific heat input of between 18 and 24kJ/cm. PWHT is, on the one hand, a parameter originating from API RP 934 A, but also results, on the other hand, from the steel users’ empirical data and their procedure qualification tests. Still more players are now involved in the discussion during definition of processing parameters: firstly, the weld filler material manufacturer, who generally tends toward higher HP values, in order that the required toughness values can be achieved in the weld. Secondly, the steel producer, whose aim is to avoid excessively stringent heat treatment. 

A total of six cases, resulting from the combination of three different HP values and two different heat-input rates, were examined. The Hollomon Parameter (HP)6 concept is used to describe the various heat treatments, such as ISR, PWHT and tempering. Identical HP values have identical effects on mechanical properties, irrespective of the individual parameters. The steel producer uses metallurgical models to calculate the effects on mechanical properties.

The parameters involved in the case studies are shown in Table 1. The HP value stated corresponds in each case to one PWHT cycle with no other previous heat treatment occurring prior to/during welding or applied by the steel producer.

The increase in the HP value for the complete heat-treatment chain is shown for Case Study 4 by way of example in Fig. 1. The steel is delivered in ‘quenched + tempered’ condition, and tempering at the heavy-plate producer’s is incorporated into calculation of the HP value. This value rises to 21.17 with every further heat treatment operation; this includes tempering, ISR and four PWHT cycles.

The results of the Charpy V-notch impact tests for Cases 1, 3, 5 and 6 are all at a high level (averages of three samples 100 to 170 J) compared to the specified 55J. This can be explained by a tempering effect due to the higher heat input. The results for Case Studies 2 and 4 do not meet the requirements. Case Study 4 has an HP value of 20.51, the same as Case Studies 1, 3 and 5. The excessively low toughnesses can therefore be attributed to the lower specific heat input. Welding was performed with lower specific heat input, and heat treatment with a lower HP, in Case No. 2.  

Figures 2 and 3 show the effects of heat treatment on the mechanical properties of the parent material. The mechanical strength values (dots) drop within the prediction bands (red and blue band) until, at some point, they fall below the minimum requirements. It must be noted that mechanical strengths in delivery condition (HP = 20.50) must also be within the requirement limits. The toughnesses in Fig. 3 initially rise, and then drop after reaching the peak. This is explained by the fact that a certain minimum PWHT is required in order to generate the necessary microstructure. The most critical variable is tensile strength, as shown in the graphic in Fig. 2.

These investigations demonstrate that close coordination between all participants is important for the processing and welding of CrMoV steels if operationally safe and reliable plant equipment is to be fabricated from them. The industry is aware of this and meets its duty of care. From the steel producer’s viewpoint, attention is drawn at an appropriate point to the danger of excessive heat treatment. Metallurgical models can be used to predict what the steel will be able withstand. 

A further side-effect of these tests is the perception that heat input is capable of affecting the toughness data of the weld metal. The samples in the tests that used lower specific heat inputs either failed to meet the minimum requirements or exhibited a tendency toward lower values. It can be concluded, conversely, that lower HP values would be yielded with a higher heat input, also with beneficial effects for the parent material.
Finally, this series of tests illustrates the influence of HP values, irrespective of holding temperatures and times: “Identical value, identical influence”.l

1 API Recommended Practice 934-A [latest edition]: Materials and Fabrication of 
2 1/4Cr-1Mo, .2 1/4Cr-1Mo-1/4V, 3Cr-1Mo, and 3Cr-1Mo-1/4V Steel Heavy Wall Pressure Vessels for High-temperature, High-pressure Hydrogen Service
2 API Technical Report 934-B [latest edition]: Fabrication considerations for Vanadium-modified Cr-Mo steel heavy wall pressure vessels
3 ASME Code Section II Part A, Edition 2013, SA-387/SA-387M, Specification for pressure vessel plates, alloy steel, chromium molybdenum
4 ASME Boiler & Pressure Vessel Code
5 ASME Code Section II Part A, Edition 2013, SA-542/SA-542M, Specification for pressure vessel plates, alloy steel, quenched and tempered, chromium molybdenum and chromium molybdenum vanadium
6 Hollomon, John Herbert & JAFFE, L.D. Time-Temperature relations in Tempering Steel. Transactions of the AIME, 162, 1645, 223-249  

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Ingo Detemple & Jörg Maffert are with Dillinger Hüttenwerke, Dillinger, Germany

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