Basic metallurgy for welders

How much does a welder need to know about metallurgy? Hands-on welders don’t need to be metallurgical engineers, but they must be knowledgeable about certain chemical, mechanical, and physical properties of various materials.

In a small job shop, welders often are required to pick up a drawing and do the whole job unassisted and without a written welding procedure. The customer usually furnishes the design and the drawings. The material type is identified on the drawing or the bills of material by an ASTM, AISI, SAE, or ASME number. The welder must be able to choose the proper filler metal for the base metal that is to be welded.

Selection charts are available in many code books and from filler metal suppliers. These charts usually state the yield or tensile strength (mechanical metallurgy), but they do not show the chemical composition (chemical metallurgy). They also fail to provide the heat and electrical conductivity information (physical metallurgy), but this information is important to know, especially when welding aluminum or copper and their alloys. Therefore, although these charts are valuable to welders, they do not necessarily provide enough information to successfully perform a sound weld.

Chemical Metallurgy Examples

If the material is quenched and tempered (Q&T), the preheat (Figure 1) and interpass temperatures are extremely critical. The welder needs to be aware of the possibility and consequences of hydrogen entrapment.

Figure 1

Familiarity with these terms requires studying the elements involved in welding. This information, referred to as chemical metallurgy, is an important part of the bank of knowledge required in the welding industry.

If a welder understands H₂O, he knows that when water appears on the surface of metal as it is heated, the H (hydrogen) in the compound is the element that causes cracking if it is trapped in the weld. He must use a preheat chart to determine the amount of heat that the welding industry has established for diffusing the hydrogen. Some codes (such as AWS D1.1) contain these charts.

The welder also must know that hydrogen entrapment often causes delayed cracking and that the weld should be inspected (or reinspected) 24 to 48 hours after completion.

Job shop welders should be taught to read the material test report (MTR) provided by the base metal supplier. The chemical and mechanical properties are included in this document.

Sometimes the tensile and yield information is accompanied by the elongation, reduction of area, and impact values. These are mechanical properties, and this information is valuable for determining the specific type of welding material and pre- and postheating procedures required.

If the material is identified by an AISI/SAE designation, welders should learn what these numbers mean. For example, material designated 1040 requires some special treatment that is uncommon for some mild (low-carbon) steel, such as ASTM A-36. The 10 identifies the metal as a carbon steel, but the amount of carbon is much higher than that contained in the A-36 variety. (The maximum carbon content is 0.29 percent in most A-36.)

The 40 denotes the amount of carbon contained in the material by weight. The higher the carbon content is above 0.30 percent, the more care must be taken when welding, preheating, and postheating. Carbon content in excess of 0.30 percent tends to cause embrittlement. Material also seems to absorb more hydrogen as the carbon content increases.

Preheat requirements for higher-carbon-content material in any thickness must be closely adhered to. Some engineers and welding technicians believe that this material should not be welded because it is susceptible to cracking. If extreme care is taken, it can be welded successfully. A welder who is familiar with all the requirements is likely to achieve crack-free welds consistently on this material.

Other elements that can cause severe problems when welding are manganese and sulfur. When these two elements combine, they can form manganese sulfides, which are the most common causes of lamination or delamination.

Laminar inclusions almost always form in the center of the through thickness of the material. Sometimes the inclusions cannot be detected without ultrasonic examination, but welders should observe the manganese and sulfur content stated in the MTR. If the content of both of these elements is higher than normal (manganese above 1.60 percent or sulfur at 0.005 percent or higher), the possibility of manganese sulfide formation is greater. A recommended precaution is to perform an ultrasonic examination (straight through transmission) for any carbon steel material that is 1 in. thick or thicker before beginning any fabrication. Once fabrication (cutting, bending, etc.) is performed, the problem becomes the fabricator’s. The mill or distributor will replace only the laminated material. The fabricator will not be reimbursed for the labor or welding materials.

Mechanical Metallurgy Examples

Engineers must establish the strength and ductility values for the part to be fabricated. The engineer also is responsible for selecting the most desirable material for the fabricated product’s end use, based on criteria such as indoor-outdoor use, corrosion considerations, or aesthetics.

These values also are important to the job shop welder, who will use them to determine the filler material, the welding process to use, and the pre- and postweld heating procedures.

A good example is welding weathering steel. The first of this type of material was called CORE-TEN®, which was a trade name for ASTM A-588. This material became popular because it does not require a coating in a normal outdoor atmosphere. A copper oxide is formed on the surface of the material to prevent deep rusting. The welder not only needs to know that it is copper-bearing, which calls for caution with the temperature (note the lower requirements on the preheat chart), but also that it has yield strength of 50 kilopounds per square inch (KSI).

Just observing the appearance of the material before the oxidation sets in is not enough to differentiate it from plain A-36, which has yield strength of only 36 KSI. This is another instance that requires welders to study the MTR. The welder must select a welding material that matches both the steel’s mechanical properties and the atmospheric corrosion properties as closely as possible.

Until very recently no filler metal was produced specifically for weathering steel. The matching filler metal charts proposed the use of low-hydrogen electrodes, E7018 or E7028. These electrodes did not provide the atmospheric corrosion resistance properties. The industry’s choices ran from ER80S-B2 to ER80S-Ni-2 or for FCAW, E81T1-Ni2. The ER80S-B2 possesses the necessary atmosphere corrosion properties, but the mechanical properties—toughness and ductility—are lacking. The Ni-2 fillers are a bit high in the tensile and yield area, but the toughness makes up for it.

A knowledgeable welding supply company often is a very good source for providing information such as that needed for meeting multiple requirements. Web sites can be very helpful, too, but many times a product is being sold, rather than providing needed technical information. Welders should always seek out several resources and compare the information. The AWS Welding Handbooks also are very helpful.

Physical Metallurgy Examples

Physical metallurgy is concerned with material characteristics such as the conductivity of heat and electrical current; hardness; hardenability by heat treating and other methods; corrosion resistance; and magnetic permeability or retentivity.

Having this information is especially important when welding copper, aluminum, and their alloys. The heat and electrical conductivity are very different in these materials. In gas tungsten arc welding (GTAW), high-frequency AC is used for welding the aluminum alloys and sometimes the copper alloys. Higher amperage must be used to counteract the rapid heat and electrical current conductivity. This higher amperage requires a larger tungsten electrode. Heat dissipates very rapidly in both of these materials. An arc is established by electrical resistance between the electrode and the workpiece. In copper, aluminum, and their alloys, the electrical current has a tendency to travel throughthe material without much resistance at all.

Methods for hardening these materials often are much different from those used to harden steel, stainless steel and other alloys. In most cases, the alloys must be strain- or work-hardened. This method is accomplished by stretching (reversing the stresses) or impacting the material. Welders must be aware of this because the bending or roll forming produces hardness and, therefore, more crack sensitivity.

Some aluminum alloys, such as 6061T-X, are heat-treatable. Welders should consult the experts for preheat and interpass data for these materials. Improper heating can cause softening and loss of strength. Some of the alloys are crack-sensitive, and the probability for cracking is increased by certain welding practices.

One of the main causes of weld discontinuities and defects in these materials is oxide formation on the surface. Copper material can be purchased in a deoxidized condition, which helps alleviate the problem. Aluminum alloys must be chemically cleaned before each weld is deposited. Aluminum oxide forms very rapidly and causes nonfusion or cold laps in the weld.

Oxide also can attract hydrogen. These defects and discontinuities often are not detectable with visual inspection. Welders must be aware of the potential and take necessary precautions. For multipass welds, a penetrant test is advisable between passes.

Welders should have written procedures to follow; unfortunately, this doesn’t always happen. Because of time and budget constraints, technical schools (in secondary and postsecondary educational institutions) do not teach all the necessary metallurgical information. Some community colleges have begun programs intended to address this deficiency by hiring metallurgically informed people from industry as adjunct faculty.