Weldability of Metals
All metals can be welded, although some metals require far more care and skill to produce acceptably strong and ductile joints. The term weldability has been coined to describe the ease with which a metal can be welded properly. Good weldability means that almost any process can be used to produce acceptable welds and that little effort is needed to control the procedures. Poor weldability means that the processes used are limited and that the preparation of the joint and the procedure used to fabricate it must be controlled very carefully or the weldment will not function as intended.
Knowing why a part broke is as important to the repair process as knowing the type of metal it is made with. Parts break because they are worn out, damaged in an accident, underdesigned for the work, or defective. Welders have the greatest success fixing parts that are worn out or damaged in an accident. These parts were giving good service before they broke. Parts that were underdesigned or defective are more likely to break again if they are welded without fixing the design problem or defect first. For example, a bracket that vibrates too much and cracks will continue to vibrate and will crack again if you just weld the crack. Instead, see what can be done to stop the vibration to prevent it from cracking again. In other words, fix the problem before fixing the part—or you will be fixing the part again and again.
Welding processes produce a thermal cycle in which the metals are heated over a range of temperatures. Cooling of the metal to ambient temperatures then follows. The heating and cooling cycles can set up stresses and strains in the weld. Whatever the welding process used, certain metallurgical, physical, and chemical changes also take place in the metal. A wide range of welding conditions can exist for welding methods when joining metals with good weldability. However, if weldability is a problem, adjustments usually will be necessary in one or more of the following factors:
Filler metal—If the wrong filler metal is selected, the weld can have major defects and not be fit for service. Common defects include porosity, cracks, and filler metal that just will not stick. The cracks can be in the filler metal or in the base metal alongside the weld. If you are not sure which filler metal to use, a good general rule is that a little stronger or higher grade can be used successfully, but a lower strength or grade seldom works.
Preheat and postheat—Cracking is a common problem when welding on brittle metals such as cast iron or some high-strength alloys. Preheating the part before starting the weld reduces the stress caused by the weld and helps the filler metal flow. The most commonly used preheat temperature range is between 250°F and 400°F (120°C and 200°C) for most steel. The preheat temperature can be as high as 1200°F (650°C) when welding cast iron. Preheating is required any time that the metal to be welded is below 70°F (20°C) because the cold metal quenches the weld. Postheating slows the cooldown rate following welding, which prevents postweld cracking of brittle metals. Postheating also reduces weld stresses that can result in cracks forming some time after the part is repaired.
Welding procedure—The size of the weld bead, the number of welds, and the length of the welds all affect the weld. When large welds are needed, it is better to make more small welds than a few large welds. The small welds serve to postheat the weld. They reduce stresses and result in less distortion. Sometimes a series of short back-stepping welds can be made. For example these short welds can be used on very brittle metals like cast iron.
Carbon and Alloy Steels
Steels alloyed with carbon and only a low concentration of silicon and manganese are known as plain carbon steels. These steels can be classified as low carbon, medium carbon, and high carbon steels. The division is based upon the percentage of carbon present in the material.
Plain carbon steel is basically an alloy of iron and carbon. Small amounts of silicon and manganese are added to improve its working quality. Sulfur and phosphorus are present as undesirable impurities. All steels contain some carbon, but steels that do not include alloying elements other than low levels of manganese or silicon are classified as plain carbon steels. Alloy steels contain specified larger proportions of alloying elements.
The AISI has adopted the following definition of carbon steel: “Steel is classified as carbon steel when no minimum content is specified or guaranteed for aluminum, chromium, columbium, molybdenum, nickel, titanium, tungsten, vanadium, or zirconium; and when the minimum content of copper which is specified or guaranteed does not exceed 0.40%; or when the maximum content which is specified or guaranteed for any of the following elements does not exceed the respective percentages hereinafter stated: manganese 1.65%, silicon 0.60%, copper 0.60%.” Under this classification will be steels of different composition for various purposes.
Many special alloy steels have been developed and sold under various trade names. These alloy steels usually have special characteristics, such as high tensile strength, resistance to fatigue, corrosion resistance, or the ability to perform at high temperatures. Basically, the ability of carbon steel to be welded is a function of the carbon content. (Other factors to be considered include thickness and the geometry of the joint.) All carbon steels can be welded by at least one method. However, the higher the carbon content of the metal, the more difficult it is to weld the steel. Special instructions must be followed in the welding process.
Low Carbon and Mild Steel
Low carbon steel has a carbon content of 0.15% or less, and mild steel has a carbon content range of 0.15 to 0.30%. Both steels can be welded easily by all welding processes. The resulting welds are of method. The selection of the correct electrode for the particular welding application helps to ensure high strength and ductility in the weld.
The gas metal arc (GMA) and flux cored arc welding processes are used for welding both low and medium carbon steels due to the ease of welding and because they prevent contamination of the weld. The high productivity and lower cost make them increasingly popular welding processes.
Medium Carbon Steel
The welding of medium carbon steels, having 0.30 to 0.50% carbon content, is best accomplished by the various fusion processes, depending upon the carbon content of the base metal. The welding technique and materials used are dictated by the metallurgical characteristics of the metal being welded. For steels containing more than 0.40% carbon, preheating and subsequent heat treatment are generally required to produce a satisfactory weld. Stick welding electrodes of the type used on low carbon steels can be used for welding this type of steel. The use of an electrode with a special low hydrogen coating may be necessary to reduce the tendency toward underbead cracking.
High Carbon Steel
High carbon steels usually have a carbon content of 0.50 to 0.90%. These steels are much more difficult to weld than either the low or medium carbon steels. Because of the high carbon content, the heat-affected zone next to the weld can become very hard and brittle. You can avoid this by using preheating and by selecting procedures that produce high-energy inputs to the weld. The martensite that does form is tempered by postweld heat treatments such as stress relief anneal.
In arc welding high carbon steel, mild-steel shielded arc electrodes are generally used. However, the weld metal does not retain its normal ductility because it absorbs some of the carbon from the steel.
Welding on high carbon steels is often done to build up a worn surface to original dimensions or to develop a hard surface. In this type of welding, preheating or heat treatment may not be needed if heat builds up in the part during continuous welding.
Stainless Steels
Stainless steels consist of four groups of alloys: austenitic, ferritic, martensitic, and precipitation hardening. The austenitic group is by far the most common. Its chromium content provides corrosion resistance, while its nickel content produces the tough austenitic microstructure. These steels are relatively easy to weld, and a large variety of electrode types are available.
The most widely used stainless steels are the chromium-nickel austenitic types. They are used for items such as dairy equipment, including milk tanks; hand sprayer tanks; and poultry medicine infusion system pumps; and are usually referred to by their chromium-nickel content as 18/8, 25/12, 25/20, and so on. For example, 18/8 contains 18% chromium and 8% nickel, with 0.08 to 0.20% carbon. To improve weldability, the carbon content should be as low as possible. Carbon should not be more than 0.03%, with the maximum being less than 0.10%.
Keeping the carbon content low in stainless steel will also help reduce carbide precipitation. Carbide precipitation occurs when alloys containing both chromium and carbon are heated. The chromium and carbon combine to form chromium carbide (Cr3C2).
The combining of chromium and carbon lowers the chromium that is available to provide corrosion resistance in the metal. This results in a metal surrounding the weld that will oxidize or rust. The amount of chromium carbide formed is dependent on the percentage of carbon, the time that the metal is in the critical range, and the presence of stabilizing elements.
If the carbon content of the metal is very low, little chromium carbide can form. Some stainless steel types have a special low carbon variation. These low carbon stainless steels are the same as the base type but with much lower carbon content. To identify the low carbon from the standard AISI number, the L is added as a suffix. See examples 304 and 304.
Chromium carbides form when the metal is between 800°F and 1500°F (625°C and 815°C). The quicker the metal is heated and cooled through this range, the less time that chromium carbides can form. Since austenitic stainless steels are not hardenable by quenching, the weld can be cooled using a chill plate. The chill plate can be water-cooled for larger welds.
Some filler metals have stabilizing elements added to prevent carbide precipitation. Columbium and titanium are both commonly found as chromium stabilizers. Examples of the filler metals are E310Cb and ER309Cb.
In fusion welding, stainless austenitic steels may be welded by all of the methods used for plain carbon steels.
Since ferritic stainless steels contain almost no nickel, they are cheaper than austenitic steels. They are used for ornamental or decorative applications such as architectural trim and at elevated temperatures for heat exchanging. However, ferritic stainless steels also tend to be brittle unless specially deoxidized. Special high-purity, high-toughness ferritic stainless steels have been developed, but careful welding procedures must be used with them to prevent embrittlement. This means very carefully controlling nitrogen, carbon, and hydrogen.
Martensitic stainless steels are also low in nickel but contain more carbon than the ferritic. They are used in applications requiring both wear resistance and corrosion resistance. Items such as surgical knives and razor blades are made of them. Quality welding requires very careful control of both preheating and tempering immediately after welding.
Precipitation hardening stainless steels can be much stronger than the austenitic, without losing toughness. Their strength is the result of a special heat treatment used to develop the precipitate. They can be solution treated prior to welding and given the precipitation treatment after welding.
The closer the characteristics of the deposited metal match those of the material being welded, the better is the corrosion resistance of the welded joint. The following precautions should be noted:
Any carburization or increase in carbon must be avoided, unless a harder material with improved wear resistance is actually desired. In this case, there will be a loss in corrosion resistance.
It is important to prevent all inclusions of foreign matter, such as oxides, slag, or dissimilar metals.
In welding with the metal arc process, direct current is more widely used than alternating current. Generally, reverse polarity is preferred where the electrode is positive and the workpiece is negative. The diameter of the electrode used to weld steel that is thinner than 3/16 in. (4.8 mm) should be equal to, or slightly less than, the thickness of the metal to be welded.
When setting up for welding, material 0.050 in. (1.27 mm) and less in thickness should be clamped firmly to prevent distortion or warpage. The edges should be butted tightly together. All seams should be accurately aligned at the start. It is advisable to tack weld the joint at short intervals as well as to use clamping devices.
The electrode should always point into the weld in a backhand or drag angle. Avoid using a figure-eight pattern or an excessive side weaving motion such as that used in welding carbon steel. Best results are obtained with a stringer bead with a little or very slight weaving motion and steady forward travel, with the electrode holder leading the weld pool at about 60° in the direction of travel. To weld stainless steels, the arc should be as short as possible.
Aluminum Weldability
One of the characteristics of aluminum and its alloys is that it has a great affinity for oxygen. Aluminum atoms combine with oxygen in the air to form an oxide with a high melting point that covers the surface of the metal. This feature, however, is the key to the high resistance of aluminum to corrosion. It is because of this resistance that aluminum can be used in applications where steel is rapidly corroded.
Pure aluminum melts at 1200°F (650°C). The oxide that protects the metal melts at 3700°F (2037°C). This means that the oxide must be cleaned from the metal before welding can begin.
When the GMA welding process is used, the stream of inert gas covers the weld pool, excluding all air from the weld area. This prevents reoxidation of the molten aluminum. GMA welding does not require a flux.
Aluminum can be arc welded using aluminum welding rods. These rods must be kept in a dry place because the flux picks up moisture easily. Because aluminum melts so easily, use a piece of clean steel plate as a backing to weld on thin sections. The steel backing plate can support the root of the weld without the aluminum weld sticking to the steel plate. Thick aluminum casting must be preheated to about 400°F (200°C) before welding. The preheating helps the weld flow and reduces weld spatter.
Aluminum has high thermal conductivity. Aluminum and its alloys can rapidly conduct heat away from the weld area. For this reason, it is necessary to apply the heat much faster to the weld area to bring the aluminum to the welding temperature. Therefore, the intense heat of the electric arc makes this method best suited for welding aluminum.
When aluminum welds solidify from the molten state, they will shrink about 6% in volume. The stress that results from this shrinkage may create excessive joint distortion unless allowances are made before joining the metal. Cracking can occur because the thermal contraction is about two times that of steel. The heated parent metal expands when welding occurs. This expansion of the metal next to the weld area can reduce the root opening on butt joints during the process. The contraction that results upon cooling, plus the shrinkage of the weld metal, creates tension and increases cracking.
The shape of the weld groove and the number of beads can affect the amount of distortion. Less distortion occurs with two-pass square butt welds. Other factors that have an influence on the weld are the speed of welding, the use of properly designed jigs and fixtures to support the aluminum while it is being welded, and tack welding to hold parts in alignment.
SUMMARY
All metals are weldable. The only limitation in the fabrication and repair of parts is the cost. It takes a skilled welder with an understanding of all the various characteristics of the metals and types of welding to fix worn or damaged parts. Being able to recognize the differences among the various classifications of metal will allow you to select the most appropriate welding repair procedure. Because of the complexity of this process, you may often be required to research through the original manufacturer of the equipment the types of processes and materials used in the weldment’s construction.
Processes change, so sometimes after a part is placed in service, there is a need to change the welding procedures used in the part’s original construction for the repair welding. In addition, the original manufacturer may no longer have the welding procedure. To make a successful weld in such cases, you must be able to establish a new welding procedure. As part of your welding procedure, you may perform tests to enhance the longevity of your repairs. Your welding experience will help you to be more efficient in producing the new welding procedure.
Repair welds do not have to look pretty; they only have to hold. Years ago, when the author was welding in a small agricultural welding shop in Madisonville, Tennessee, a local farmer said, “Go on and try and weld it. It’s broken anyway, and if you can’t fix it it’s no use to me anyway.” He was right—if you have a broken part, and it is going to cost more to have someone else weld it than to buy a new part, you might as well try welding it yourself. You have nothing to lose and everything to gain; if you fix it, you are in luck, and if you burn it up, you would have had to buy a replacement part anyway. Make sure you can get a replacement part before you start welding on the broken one. If the original part is damaged during the weld, a professional might not be able to repair it. Last, a tendency of many welders is to overweld when making a repair. Keeping the weld sized correctly will make the part stronger. A weld that is too large can cause the part to be brittle and break.