THE INDIAN INSTITUTE OF WELDING - MUMBAI
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Weldability of materials Nickel and nickel alloys Nickel and nickel based alloys are chosen because of their: corrosion resistance heat resistance and high temperature properties low temperature properties Types of nickel alloys are identified and guidance is given on welding processes and techniques which can be used in fabricating nickel alloy components without impairing their corrosion or mechanical properties or introducing defects into the weld. Material types The alloys can be grouped according to the principal alloying elements. Although there are National and International designations for the alloys, trade names such as Inconel and Hastelloy are more commonly used. In terms of their weldability, these alloys can be classified according to the means by which the alloying elements develop the mechanical properties, namely solid solution alloys and precipitation hardened alloys. A distinguishing feature of precipitation hardened alloys is that mechanical properties are developed by heat treatment (solution treatment plus ageing) to produce a fine distribution of hard particles in a nickel rich matrix. Solid solution alloys Solid solution alloys are Nickel 200, Monel alloy 400 series, Inconel alloy 600 series, Incoloy alloy 800 series, Hastelloys and some Nimonic alloys (such as 75, and PE13). These alloys are readily fusion welded, normally in the annealed condition. As the heat affected zone (HAZ) does not harden, heat treatment is not usually required after welding. Precipitation hardening alloys Precipitation hardened alloys are the Monel alloy 500 series, Inconel alloy 700 series, Incoloy alloy 900 series and most of the Nimonic alloys (such as 80,90,263 and PE16). These alloys may susceptible to post-weld heat treatment cracking. Weldability Most nickel alloys can be fusion welded using gas shielded processes like TIG or MIG. Of the flux processes, MMA is frequently used but the SAW process is restricted to solid solution alloys (Nickel 200, Inconel alloy 600 series and Monel alloy 400 series) and is less widely used. Solid solution alloys are normally welded in the annealed condition and precipitation hardened alloys in the solution treated condition. Preheating is not necessary unless there is a risk of porosity from moisture condensation. It is recommended that material containing residual stresses be solution-treated before welding to relieve the stresses. Post-weld heat treatment is not usually needed to restore corrosion resistance but thermal treatment may be required for precipitation hardening or stress relieving purposes to avoid stress corrosion cracking. Filler alloys Filler composition normally matches the parent metal. However, most fillers contain a small mount of titanium, aluminium and/or niobium to help minimise the risk of porosity and cracking. Filler metals for gas shielded processes are covered in BS 2901: Pt 5 and in the USA by AWS A5.14. Recommended fillers for selected alloys are given in the table. Table 1: Filler selection for nickel alloys Filler designations Alloy BS 2901:Part 5 AWS A5.14 Tradenames Comments Pure nickel Nickel 200 NA32 ERNi-3 Nickel 61 'Matching' filler metal normally contains 3%Ti Nickel copper Monel 40 NA33 ERNiCu-7 Monel 600 'Matching' filler metal contains additions of Mn, Ti and Al Nickel chromium Brightray S NA34 - NC 80/20 Ni-Cr and Ni-Cr-Fe filler metals may be used Nimonic 75 NA34 - NC 80/20 Nickel-chromium-iron Incoloy 800 NA43 ERNiCrMo-3 Inconel 625 Thermanit 21/33 Usually welded with Ni-Cr-X alloys, but more nearly matching consumables are available which contain higher C and also Nb Inconel 600 NA35 ERNiCr-3 Inconel 82 'Matching' filler metal contains Nb addition Inconel 718 NA51 ERNiFeCr-2 Inconel 718 'Matching' filler metal can be used but Inconel 625 is an alternative consumable offering increased crack resistance Nickel-chromium-molybdenum Inconel 625 NA43 ERNiCrMo-3 Inconel 625 'Matching'Inconel 625 filler metal is also used widely (for cladding and dissimilar welds) Hastelloy C-22 - ERNiCrMo-10 Hastelloy C-22 Nickel-molybdenum Hastelloy B-2 NA44 ERNiMo-7 Hastelloy B-2 Corrosion resistant alloys require matching fillers High temperature alloys Hastelloy-X Waspaloy NA40 ERNiCrMo-2 Hastelloy X Waspaloy Mechanical properties required in joints dictate whether matching precipitation hardening fillers or solid solution alloys such as Inconel 625 are used Imperfections and degradation Nickel and its alloys are readily welded but it is essential that the surface is cleaned immediately before welding. The normal method of cleaning is to degrease the surface, remove all surface oxide by machining, grinding or scratch brushing and finally degrease. Common imperfections found on welding are: porosity oxide inclusions and lack of inter-run fusion weld metal solidification cracking microfissuring Additionally, precautions should be taken against post-welding imperfections such as: post-weld heat treatment cracking stress corrosion cracking Porosity Porosity can be caused by oxygen and nitrogen from air entrainment and surface oxide or by hydrogen from surface contamination. Careful cleaning of component surfaces and using a filler material containing deoxidants (aluminium and titanium) will reduce the risk. When using argon in TIG and MIG welding, attention must be paid to shielding efficiency of the weld pool including the use of a gas backing system. In TIG welding, argon-H2 gas mixtures which provide a slightly reducing atmosphere are particularly effective. Oxide inclusions and lack of inter-run fusion As the oxide on the surface of nickel alloys has a much higher melting temperature than the base metal, it may remain solid during welding. Oxide trapped in the weld pool will form inclusions. In multi-run welds, oxide or slag on the surface of the weld bead will not be consumed in the subsequent run and will cause lack of fusion imperfections. Before welding, surface oxide, particularly if it has been formed at a high temperature, must be removed by machining or abrasive grinding; it is not sufficient to wire brush the surface as this serve only to polish the oxide. During welding, surface oxide and slag must be removed between runs. Weld metal solidification cracking Weld metal or hot cracking results from contaminants concentrating at the centreline and an unfavourable weld pool profile. Too high a welding speed produces a shallow weld pool which encourages impurities to concentrate at the centreline and, on solidification, generates sufficiently large transverse stresses to form cracks. This risk can be reduced by careful cleaning of the joint area and avoiding high welding speeds. Microfissuring Similar to austenitic stainless steel, nickel alloys are susceptible to formation of liquation cracks in reheated weld metal regions or parent metal HAZ. This type of cracking is controlled by factors outside the control of the welder such as grain size or content impurity. Some alloys are more sensitive than others. For example, the extensively studied Inconel 718 is now less sensitive than some cast superalloys which cannot be welded without inducing liquation cracks. Post-weld heat treatment cracking This is also known as strain-age or reheat cracking. It is likely to occur during post-weld ageing of precipitation hardening alloys but can be minimised by pre-weld heat treatment. Solution annealing is commonly used but overageing gives the most resistant condition. Inconel 718 alloy was specifically developed to be resistant to this type of cracking. Stress corrosion cracking Welding does not normally make nickel alloys susceptible to weld metal or HAZ corrosion. However, when the material will be in contact with caustic soda, fluosilicates or HF acid, stress corrosion cracking is possible. After welding, the component or weld area must be given a stress-relieving heat treatment to prevent stress corrosion cracking. Copyright by TWI, 1999 About the IIW / Mumbai Branch / Other Branches / Coming Events / Technical Lectures Mumbai Weldnet / Trends in welding / Related Websites / IIW Forum / Feedback / Home