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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. Six cases, resulting from the combination of three
Fig. 3. The influence of HP value on the toughnesses of a CrMoV plate in the 130 to 160mm thickness range.
different HP values and two different heat-input rates, were examined. Te 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. Te steel producer uses metallurgical models to calculate the effects on mechanical properties.
calculation of the HP value. Tis value rises to 21.17 with every further heat treatment operation; this includes tempering, ISR and four PWHT cycles. Te 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. Tis can be explained by a tempering effect due to the higher heat input. Te 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 3, 5 and 6. Te 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. Te 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. Te toughnesses in Fig. 3 initially rise, and then drop after reaching the peak. Tis is explained by the fact that a certain minimum PWHT is required in order to generate the necessary microstructure. Te most critical variable is tensile strength, as shown in the graphic in Fig. 2. Tese investigations demonstrate that close
Te parameters involved in the case studies are shown below. Te 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. Te increase in the HP value for the complete
heat-treatment chain is shown for Case Study 4 by way of example in Fig. 1. Te steel is delivered in ‘quenched + tempered’ condition, and tempering at the heavy-plate producer’s is incorporated into
References 1
API Recommended Practice 934-A [latest edition]: Materials and Fabrication of 2¼Cr-1Mo, .2¼Cr-1Mo-¼V, 3Cr-1Mo, and 3Cr-1Mo-¼V Steel Heavy Wall
Pressure Vessels for High-temperature, High-pressure Hydrogen Service 2
Vanadium-modified Cr-Mo steel heavy wall pressure vessels 3
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
API Technical Report 934-B [latest edition]: Fabrication considerations for ASME Code Section II Part A, Edition 2013, SA-387/SA-387M, Specification for
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. Te 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. Te 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
For more information ✔ at
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Hans-Willi Bonn, Ingo Detemple & Jörg Maffert are with Dillinger Hütte in Germany.
www.dillinger.de
www.engineerlive.com 37
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