Defects - hydrogen cracks in steels - prevention and best practice

Preheating of a jacket structure to prevent hydrogen cracking

In this issue, techniques and practical guidance on the avoidance of hydrogen cracks are described. Preheating, interpass and post heating to prevent hydrogen cracking There are three factors which combine to cause cracking in arc welding: hydrogen generated by the welding process a hard brittle structure which is susceptible to cracking residual tensile stresses acting on the welded joint In practice, for a given situation (material composition, material thickness, joint type, electrode composition and heat input), the risk of hydrogen cracking is reduced by heating the joint. Preheat Preheat, which slows the cooling rate, allows some hydrogen to diffuse away and prevents a hard, crack-sensitive structure being formed. The recommended levels of preheat for carbon and carbon manganese steel are detailed in BS 5135. (Nb a draft European standard Pr EN 1011-2 is expected to be introduced in 2000). The preheat level may be as high as 200°C for example, when welding thick section steels with a high carbon equivalent (CE) value. Interpass and post heating As cracking rarely occurs at temperatures above ambient, maintaining the temperature of the weldment during fabrication is equally important. For susceptible steels, it is usually appropriate to maintain the preheat temperature for a given period, typically between 2 to 3 hours, to enable the hydrogen to diffuse away from the weld area. In crack sensitive situations such as welding higher CE steels or under high restraint conditions, the temperature and heating period should be increased, typically 250-300°C for three to four hours. Post weld heat treatment (PWHT) may be used immediately on completion of welding ie without allowing the preheat temperature to fall. However, in practice, as inspection can only be carried out at ambient temperature, there is the risk that 'rejectable,' defects will only be found after PWHT. Also, for highly hardenable steels, a second heat treatment may be required to temper the hard microstructure present after the first PWHT. Under certain conditions, more stringent procedures are needed to avoid cracking than those derived from the nomograms for estimating preheat in BS 5135. Appendix E of this standard mentions the following conditions: high restraint thick sections ( approximately 50mm) low carbon equivalent steels (CMn steels with C 0.1% and CE approximately 0.42) 'clean' or low sulphur steels (S approximately 0.008%), as a low sulphur and low oxygen content will increase the hardenability of a steel. alloyed weld metal where preheat levels to avoid HAZ cracking may be insufficient to protect the weld metal. Low hydrogen processes and consumables should be used. Schemes for predicting the preheat requirements to avoid weld metal cracking generally require the weld metal diffusible hydrogen level and the weld metal tensile strength as input. Use of austenitic and nickel alloy weld metal to prevent cracking In situations where preheating is impractical, or does not prevent cracking, it will be necessary to use an austenitic consumable. Austenitic stainless steel and nickel electrodes will produce a weld metal which at ambient temperature, has a higher solubility for hydrogen than ferritic steel. Thus, any hydrogen formed during welding becomes locked in the weld metal with very little diffusing to the HAZ on cooling to ambient. A commonly used austenitic MMA electrode is 23Cr:12Ni (eg from BS 2926:1984). However, as nickel alloys have a lower coefficient of thermal expansion than stainless steel, nickel austenitic electrodes are preferred when welding highly restrained joints to reduce the shrinkage strain. Figure 1 is a general guide on the levels of preheat when using austenitic electrodes. When welding steels with up to 0.2%C, a preheat would not normally be required. However, above 0.4%C a minimum temperature of 150°C will be needed to prevent HAZ cracking. The influence of hydrogen level and the degree of restraint are also illustrated in the figure. Fig.1 Guide to preheat temperature when using austenitic MMA electrodes at 1-2kJ/mm a) low restraint (e.g. material thickness <30mm) b) high restraint (e.g. material thickness >30mm) Best practice in avoiding hydrogen cracking Reduction in weld metal hydrogen The most effective means of avoiding hydrogen cracking is to reduce the amount of hydrogen generated by the consumable, ie by using a low hydrogen process or low hydrogen electrodes. Welding processes can be classified as very low, low, medium or high depending on the amount of weld metal hydrogen produced: Very low <5ml/100g Low 5 - 10ml/100g Medium 10 - 15ml/100g High >15ml/100g Figure 2 illustrates the relative amounts of weld metal hydrogen produced by the major welding processes. MMA, in particular, has the potential to generate a wide range of hydrogen levels. Thus, to achieve the lower values, it is essential that basic electrodes are used and they are baked in accordance with the manufacturer's recommendations. For the MIG process, cleaner wires will be required to achieve very low hydrogen levels. Fig.2 General relationships between potential hydrogen and weld metal hydrogen levels for arc welding processes General guidelines The following general guidelines are recommended for the various types of steel but requirements for specific steels should be checked according to BS 5135 or BS EN 1011: Mild steel (CE <0.4) - readily weldable, preheat generally not required if low hydrogen processes or electrodes are used - preheat may be required when welding thick section material, high restraint and with higher levels of hydrogen being generated C-Mn, medium carbon, low alloy steels (CE 0.4 to 0.5) - thin sections can be welded without preheat but thicker sections will require low preheat levels and low hydrogen processes or electrodes should be used Higher carbon and alloyed steels (CE >0.5) - preheat, low hydrogen processes or electrodes, post weld heating and slow cooling required. More detailed guidance on the avoidance of hydrogen cracking is described in BS 5135. Practical Techniques The following practical techniques are recommended to avoid hydrogen cracking: clean the joint faces and remove contaminants such as paint, cutting oils, grease use a low hydrogen process if possible dry the electrodes (MMA) or the flux (submerged arc) in accordance with the manufacturer's recommendations reduce stresses on the weld by avoiding large root gaps and high restraint if preheating is specified in the welding procedure, it should also be applied when tacking or using temporary attachments preheat the joint to a distance of at least 75mm from the joint line ensuring uniform heating through the thickness of the material measure the preheat temperature on the face opposite that being heated. Where this is impractical, allow time for the equalisation of temperature after removing the preheating before the temperature is measured adhere to the heat input requirements maintain heat for approximately two to four hours after welding depending on crack sensitivity In situations where adequate preheating is impracticable, or cracking cannot be avoided, austenitic electrodes may be used Acceptance standards As hydrogen cracks are linear imperfections which have sharp edges, they are not permitted for welds meeting the quality levels B, C and D in accordance with the requirements of BS EN 25817 (ISO 5817). Detection and remedial action As hydrogen cracks are often very fine and may be sub-surface, they can be difficult to detect. Surface-breaking hydrogen cracks can be readily detected using visual examination, liquid penetrant or magnetic particle testing techniques. Internal cracks require ultrasonic or radiographic examination techniques. Ultrasonic examination is preferred as radiography is restricted to detecting relatively wide cracks parallel to the beam. Most codes will specify that all cracks should be removed. A cracked component should be repaired by removing the cracks with a safety margin of approximately 5mm beyond the visible ends of the crack. The excavation is then re-welded. To make sure that cracking does not re-occur, welding should be carried out with the correct procedure, ie preheat and an adequate heat input level for the material type and thickness. However, as the level of restraint will be greater and the interpass time shorter when welding within an excavation compared to welding the original joint, it is recommended that a higher level of preheat is used (typically by 50°C). References BS 5135:1984 Arc Welding of Carbon and Carbon Manganese Steels Pr EN 1011-1:1998 Welding - Recommendations for Welding of Metallic Materials Part 1- General Guidance for Arc Welding Part 2- Arc Welding of Ferritic Steels BS EN ISO 13916: 1997 Welding - Guidance on the Measurement of Preheating Temperature, Interpass Temperature and Preheat Maintenance Temperature N Bailey et al, Welding steels without hydrogen cracking, Woodhead Publishing, 1993 Copyright 2000, TWI Ltd About the IIW / Mumbai Branch / Other Branches / Coming Events / Technical Lectures Mumbai Weldnet / Trends in welding / Related Websites / IIW Forum / Feedback / Home