Guidelines for Welding Dissimilar Metals
Total Page：16
File Type：pdf, Size：1020Kb
Pay attention to dissimilar-metal welds Guidelines for welding dissimilar metals Reprinted with permission from Chemical Engineering Progress May 1991 ©1991 American Institute of Chemical Engineers All rights reserved NiDI NICKEL DEVELOPMENT INSTITUTE NiDI Reprint Series NO 14 018 Richard E. Avery The material presented in this publication has been prepared for the general information of the reader and should not be used or relied on for specific applications without first securing competent advice. The Nickel Development Institute, its members, staff and consultants do not represent or warrant its suitability for any general or specific use and assume no liability or responsibility of any kind in connection with the information herein. Pay Attention to Dissimilar-Metal Welds issimilar-metal welding refers to (FCAW), and gas tungsten arc (GTAW). Recent the joining of two different alloy With these processes there is a well-defined systems. Actually all fusion weld that preferably contains a substantial experience with D welds are dissimilar-metal welds filler-metal addition. With the GTAW boiler tubing (DMWs) because the metals being joined process, however, the amount of filler have a wrought structure and the welds added is controlled by the welder. The reveals how have a cast structure. Frequently the match- welder should be trained to make the prop- welding ing-composition filler metal is deliberately er filler-metal addition used for the partic- altered from that of the base alloys. For this ular welding procedure. practices affect discussion a dissimilar-metal weld will be 2. Low-dilution welds. Low-dilution weld joint that between metals of two different alloy welds include electron beam, laser, and systems. pulsed arc; the amount of base metal melt- performance On this matter, the chemical process ed is relatively small, and filler metals are in service. industries can learn something from the not normally added. power industry. A very common DMW ap- 3. Nonfusion joining: Typical nonfusion plication is joining ferritic [e.g., 2 1/4% joining processes are friction welding, and Cr-1% Mo (UNS K21590)] tubes to explosion welding, diffusion bonding austenitic boiler tubes such as 304H along with brazing and soldering. (S30409) or a similar austenitic stainless Dissimilar-metal joints can usually be steel. Because these welds are so impor- made by any of these methods, but low-di- tant, they are treated separately in this lution and nonfusion joining processes are article. more often used for high-production, spe- cial-application joining. DMWs encoun- Metallugical factors tered in power and process industries are Richard E. Avery, In dissimilar-metal welding, the prop- most often fusion welds made by the more Avery Consulting erties of three metals must be considered: common welding processes. Associates, Inc. the two metals being joined and the filler In fusion welding, the weld metal is a metal used to join them. For example, if mixture of the two metals being joined and one of the metals being joined is welded the filler metal. In arc welds made with using preheat when welding to itself, pre- consumable electrode processes such as heat should be used in making a DMW. SMAW, GMAW, SAW, and FCAW, the Another variable might be heat input con- weld metal is well mixed or stirred by the trol. On occasion there may be a conflict in arc action and the composition is quite uni- that the optimum control for one metal is form from one area to another. By sampling undesirable for the other. In this case, a any place in the weld bead, the weld com- compromise is needed. This is one reason position is determined and weld properties the development of a DMW procedure reasonably predicted. While the bulk of the often requires more study than for a con- weld is well mixed, there is an unmixed ventional, similar-metal welding proce- zone (UMZ) at the weld interface, which is dure. a very narrow boundary layer of melted Fusion welds and other joining meth- base metal that froze before mixing with ods. The processes available for joining the weld metal. Fortunately, the UMZ is dissimilar metals are: seldom important in normal service envi- l. Fusion welds. The processes for fu- ronments but, on rare occasions, has ex- sion welds include shielded metal arc hibited selected corrosion attack. There is (SMAW), gas metal arc (GMAW), sub- also a zone of unmelted base metal that will merged arc (SAW), flux cored arc have been altered by the heat of welding. 1 This heat-affected zone (HAZ) can Table 1. Determining DMW composition influence service life. Determining weld composition. Method Advantages Limitations It is necessary to know the approxi- mate weld metal composition before the service performance can be pre- 1. Chemical analysis of Most accurate Time consuming dicted. Table 1 lists three methods of weld determination Expensive determining the weld metal compo- sition along with advantages and lim- itations. The technique for method 1 2. Approximation of Less expensive and Estimating the is obvious: metal is removed from base metal dilution usually shorter than percentage the weld and an analysis performed. by weldcross section chemical analysis often difficult in Method 2 approximates weld dilu- and composition welds such as tion by area measurement as shown calculated multipass welds in Figure 1. Method 3 uses the fol- lowing base metal dilution percent- ages for some of the common weld- 3. Approximate dilution Very fast way of Welding ing processes: figures for common estimating “rough” technique can • SMAW (covered electrode): 20 welding processes composition have a strong to 25% dilution and composition No laboratory work influence of • GMAW (spray arc): 20 to 40% calculated involved dilution in some dilution processes, e.g., • GTAW: 20 to 50% dilution GMAW, GTAW • SAW (submerged arc): 20 to 50% dilution The figures are approximate be- metal welding. In other words, it is sion and oxidation resistance equal cause the welding technique has a assumed both metals in a DMW are to the least resistant base metal being strong influence on the dilution, par- basically weldable. joined. When a DMW is in an envi- ticularly with GTAW. Dilution in the Service condition effects. A ronment where the liquid can be an SMAW process is most predictable, properly engineered DMW matches electrolyte, the weld metal should be which is an advantage in making weld properties to the service condi- cathodic to (more corrosion resis- DMWs. tions. Some of the more important tant than) both base metals. If the When the amount of dilution from factors to be considered are me- weld is anodic (less corrosion re- the base metal is determined by ei- chanical and physical sistant), it can suffer ther method 2 or 3 of Table 1, the properties and weld accelerated galvan- average percentage of a specific corrosion/oxidation re- Ductility ic corrosion. element, X, is determined by the sistance. Dissimilar-metal formula below. In this example, the Mechanical prop- comparable to combinations dilution is 15% from each base metal erties. The weld metal the metals being A and B, while the filler metal con- should be equal to or Nickel-containing tributes 70% of the weld volume. stronger than the joined is desirable, and nickel alloys are easily welded to XX = (XA)(0.15) + (XB)(0.15) + weaker material being but not always (X )(0.70) joined, although the most commercially F possible. where XX is the average percentage American Society of used metals. of element X in the weld metal, XA is Mechanical Engineers Exceptions are fu- the percentage of element X in (ASME) code allows a sion welding to alu- base metal A, XB is the percentage weld strength of 95% in some cases. minum, titanium, and most refracto- of element X in base metal B, and XF Ductility comparable to the metals ry metals and alloys. Some of the is the percentage of element X in the being joined is desirable, but not al- most commonly encountered com- filler metal F. ways possible. binations will now be discussed. Calculations are normally made Physical properties. Weld metal Steel-to-stainless steel welds for only major alloy constituents, physical properties similar to the below 800°F. These are probably the e.g., iron, chromium, nickel, copper, base metals are desirable. In joints most frequently encountered DMWs and molybdenum, while elements that are heat cycled, a gross mis- in industry, with the possible excep- such as carbon or manganese are sel- match in the coefficient of thermal tion of boiler tube welds. In devel- dom figured. Carbon is an important expansion can lead to an early ther- oping a DMW procedure, it is im- factor in the weldability of iron base mal fatigue failure. portant to note the welding alloys, but it is of no more signifi- Weld corrosion/oxidation resis- parameters normally used for each cance in a DMW than in similar tance. The weld should have corro- of the metals being joined so that 2 2. Austenitic-covered electrodes corrosion resistance and adversely R.E. AVERY, of Avery or flux-cored wires should have low affect the mechanical properties of Consulting Associates, Inc., moisture content to prevent hydro- many standard grades of stainless Londonderry, NH (603/434- gen-associated defects in the low- steel. 2625; Fax: 603/425-2542), alloy HAZ. Coating moisture levels 2. Heating unstabilized stainless and consultant to the Nickel acceptable for welding austenitic al- steels that have a carbon content of Development Institute, has loys may cause hydrogen-related 0.03% or higher can significantly re- had more than 35 years’ ex- problems such as underbead crack- duce the intergranular corrosion re- perience in the fabrication ing in the HAZ of a low-alloy steel.