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These steels are usually welded using ferritic consumables and for this combination the risk of hydrogen assisted cold cracking HACC is high.
The use of austenitic stainless steel ASS consumables has great potential to significantly improve this issue. Yet, there are no guidelines for determination of safe level of preheat for welding ferritic steels with ASS consumables.
For this reason manufacturers adopt this parameter from procedures developed for conventional ferritic consumables thus significantly limiting the benefits ASS consumables are capable to deliver.
Productivity could be further enhanced by identifying the consimiveis interpass temperature threshold, thus reducing the stand-off times. In fabrication of steel structures productivity and cost-effectiveness of the welding process significantly affect the price of the final product. Therefore, ways of improving these factors are constantly being reviewed and advanced solutions are being implemented. Common practice to avoid this defect from occurring is to employ sufficient pre or post heating of the weldment.
However, both these treatments represent an additional step in the welding soldaagem, significantly reduce productivity and increase fabrication costs.
Despite this measure the risk of HACC was still very high as it was difficult to ensure that these restricted procedures were narrowly followed in every day manufacturing practice. The use of flux-cored ASS consumable offered optimum solution to this problem as it has got much higher solubility and considerably lower diffusivity of hydrogen compared to ferritic fillers.
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However the level of preheat remained unchanged due to the lack of guidance on welding ferritic steel with ASS consumable provided by welding standards. For a given chemistry and heat treatment history this standard advises to use so called CET method. This document further states that preheat can be reduced when ASS consumable is used as a result of better strain condition and hydrogen distribution across the weldment.
Lower levels of preheat are also permissible for multi-pass welding if adequately high inter-pass temperature is maintained in between the deposition of the single passes assisting the diffusion of hydrogen out of the areas of previous passes. However, no indication of the lower safe threshold for welding ferritic steels with austenitic stainless steel consumable is provided.
Further to reduce the risk of HACC adequate inter-pass temperature in between deposition of the single passes has to be maintained. Inter-pass temperature range is typically identical with preheat. From productivity view point it is beneficial to employ highest inter-pass temperature that does not produce extensively wide areas with high degree of softening. However when designing welding procedures the selection of inter-pass temperature ensures only freedom from HACC and the upper threshold is not usually explored.
This is not necessary for conventional manual and semi-automatic welding. The customers demand to increase the production outputs indicated necessity to explore fully automatic processes utilising welding robots that can achieve very high welding speeds, thus information about the higher inter-pass temperature thresholds became desirable. Productivity could be further improved by employing metal-cored wire instead of currently used flux-cored.
Flux-cored wire produces slag deposit on the surface of the weld. In case of multi-pass welding, the slag needs to be removed before deposition of the subsequent pass. This introduces additional difficulty for automation and is disadvantageous in terms of both productivity and cost.
Additionally, the flux also has the potential to pick-up hydrogen from the surrounding atmosphere if the consumable is stored inappropriately or over a long period of time. This hydrogen can be absorbed by the weld pool, increasing the risk of HACC. Other advantages of the metal-cored wire include higher duty cycles, faster travel speeds, low fume formation and very good cost effectiveness. This makes this wire ideal candidate for automated processes. The alloy design of this steel utilises the novel low manganese approach in order to reduce centreline segregation of MnS inclusions and thus improve mechanical properties and weldability.
Metal-cored consumable AWS A5. It is a versatile gas shielded, seamless wire capable of out of position welding and exhibits enhanced productivity and wetting action when compared to equivalent solid wire. To determine the lower preheat temperature threshold, single pass fillet weld configuration was selected and is shown in Figure 2 – a.
The deposition of multiple passes can result in formation of extensively wide softened zones especially when high heat inputs or inadequately high inter-pass temperatures are employed.
Therefore to optimise the inter-pass temperature multi-pass corner joint configuration shown in Figure 2 – b was used for this investigation. Any higher inter-pass temperature would not have a practical benefit especially for longer weld sections as the weld initially cools very rapidly and only during the later stages the cooling rate is significantly reduced. After completion of welding samples were prepared according to standard procedures for metallographic investigation and Vickers hardness testing.
Resultant microstructure and hence mechanical properties of the HAZ are function of chemical composition of the steel, peak temperature and cooling cycle experienced during welding. To demonstrate the effect of various levels of preheat on CGHAZ microstructure thermal profiles were acquired using technique of embedding thermocouples. Weld thermal history of multi-pass joints was acquired to provide information about effects of various inter-pass temperatures on cooling rate and cooling times of the over-tempered region of the welded joint.
For this purpose the thermocouples were spot-welded 15 mm from the edge of the plate.
To confirm safety of the proposed welding procedures with consumigeis ideally removed preheat and increased inter-pass temperatures the information about the effect of various levels of these two temperatures on the HAZ microstructure and hardness condumiveis essential. From HACC view point the most important part of the thermal history is the cooling leg that will influence resultant microstructure, diffusible hydrogen level and also the residual stresses formed as a consequence of welding process.
As mentioned previously CGHAZ is the most susceptible subzone thus the aim was to acquire thermal profile of this area. The susceptibility to HACC for various heat inputs and preheat conditions can be directly assessed based on this parameter.
The hydrogen diffusion is the most active at high temperatures but continues to take place at lower temperatures at slower rate. From data presented in Table 2 it is obvious that increasing the preheat temperature resulted in longer cooling times and slower cooling rates through the whole course of cooling.
Fundamental trend in hardness was gradual increase in across the entire weldment, with respect to decreasing the preheat temperature. This is due to the formation of harder microstructures as a result of increased cooling rate. Ferrite was also present in the form of very fine ferritic laths of upper bainite.
Lowering the preheat temperature favoured the formation of higher volume fraction of harder phases and the microstructural features became finer. The amount of martensite increased at the expense of cknsumiveis with lowering the preheat temperature. These data are particularly significant to industry and are a useful tool to gauge productivity gains during fabrication. Overall cooling time data is important in assessing time taken for cooling with respect to diffusion of hydrogen out of the weldment during the welding process.
It was observed that with an increase in inter-pass temperature, a gradual decrease in HAZ hardness was found to occur, as a result of slower cooling rates in transformation region promoting formation of softer microstructures. In addition, the HAZ widths for all inter-pass temperatures were very similar, with only slight increases of maximum 0.
Microstructure of OT region.
OFW – OXI COMBUSTíVEL by Marlon Hamasaki on Prezi
Therefore the microstructure of OT region was subjected to close examination and micrographs are presented in Figure 7. The soldayem of all investigated weldments consisted predominantly of tempered martensite. This observation is consistent with hardness data presented in previous section. To be able to confirm the safety of proposed lower preheat temperature is necessary to assess the influence preheat reduction on all three factors contributing to HACC microstructure, diffusible hydrogen and weld residual stresses.
However GGHAZ of the weldments produced employing both levels of preheat comprises of hard and brittle bainitic and martensitic constituents. Also HAZ hardness of the joints welded employing both preheats is very high, well about the level considered to be safe from HACC view point. This xonsumiveis hardness value is considered when welding utilising ferritic consumable. Slightly higher hardness obtained in the HAZ as a result of elimination of preheat is expected to be compensated by the ability of austenitic stainless steel weld metal to accommodate higher hydrogen and stress levels.
Results of previous work  revealed that weldments produced employing identical metal-cored austenitic stainless steel consumable as used in present study resulted in very low levels of diffusible hydrogen of 0. In fact previously used flux-cored ferritic consumable introduced 18 times more diffusible hydrogen into the weld pool compared to austenitic stainless steel filler.
However ASS weld metal possesses increased ability to yield and hence compensate thermal contraction resulting in limited evolution of residual stresses compared to ferritic weld metal .
Due to these phenomena residual stresses in joints welded with ASS consumable are likely to be lower compared to ferritic consumables, even though the lower preheat temperature is employed. In addition, on cooling HAZ of the parent material undergoes martensitic transformation that is accompanied by d expansion which to certain extent compensates the thermal shrinkage.
Formation of softer microstructures especially soladgem coarse-grained region could have beneficial effects with respect to reducing the potential risk of HACC. However this temperature was investigated as the worst case scenario to expand the temperature window when depositing short welds. On the other hand resulting prolonged cooling times after deposition of final pass favour the diffusion of the hydrogen and also temper rehardened HAZ areas thus reducing the risk of HACC.
These effects are likely to compensate implication resulting from employing proposed lower level of preheat for multi-pass joints. Further in case of multi-pass welding the implications of increased cooling rate resulting from preheat reduction are likely to be compensated by employing proposed higher inter-pass temperature.
The nature of an austenitic stainless steel filler has the potential to remove the preheat requirement, what would allow great benefits during welding fabrication in terms of time eoldagem costs for industry as the whole technological step of preheating could be avoided.
Employing higher inter-pass temperature would also have a great positive impact on productivity. Welding – Comparison of standardised methods for the avoidance of cold cracks. Structural steel welding – Welding of high strength quenched and tempered steelsStandards Australia.
Role of se shape deformation. Materials Science and Engineering A, All the contents of this journal, except where otherwise noted, is licensed under a Creative Commons Attribution License. Services on Demand Journal. Introduction In fabrication of steel structures productivity and cost-effectiveness of the welding process significantly affect the price of the final product.
After completion of welding samples were prepared according to standard procedures for metallographic investigation and Vickers hardness testing 3. Results To confirm safety of the proposed welding procedures with reduced ideally removed preheat and increased inter-pass temperatures the information fe the effect of various levels of these two temperatures on the HAZ microstructure and hardness is essential. Weld hardness profiles Fundamental trend in hardness was gradual increase in across the entire weldment, with respect to decreasing the preheat temperature.
Weld hardness profiles It was observed that with an increase in inter-pass temperature, a gradual decrease in HAZ hardness was found to occur, as a result of slower cooling rates in transformation region promoting formation of softer microstructures. How to cite this article.