Introduction to Metals
- Source & Manufacturing
- Classifications of Steel
- Crystalline Structure
- Heat Treatment
- Properties of Metals
- Effects of Alloying Elements
Source and Manufacturing
Metals come from natural deposits of ore in the earth’s crust. Most ores are contaminated with impurities that must be removed by mechanical and chemical means. Metal extracted from the purified ore is known as primary or virgin metal, and metal that comes from scrap is called secondary metal. Most mining of metal bearing ores is done by either open pit or underground methods. The two methods of mining employed are known as “selective” in which small veins or beds of high grade ore are worked, and “bulk” in which large quantities of low grade ore are mined to extract a high grade portion.
There are two types of ores, ferrous and nonferrous. The term ferrous comes from the Latin word “ferrum” meaning iron, and a ferrous metal is one that has high iron content. Nonferrous metals, such as copper and aluminum, are those that contain little or no iron. There is approximately 20 times the tonnage of iron in the earth’s crust compared to all other nonferrous products combined; therefore, it is the most important and widely used metal.
Aluminum, because of its attractive appearance, light weight and strength, is the next most widely used metal. Commercial aluminum ore, known as bauxite, is a residual deposit formed at or near the earth’s surface.
Some of the chemical processes that occur during steel making are repeated during the welding operation and an understanding of welding metallurgy can be gained by imagining the welding arc as a miniature steel mill.
The largest percentage of commercially produced iron comes from the blast furnace process. A typical blast furnace is a circular shaft approximately 90 to 100 feet in height with an internal diameter of approximately 28 feet. The steel shell of the furnace is lined with a refractory material, usually a hard, dense clay firebrick.
The iron blast furnace utilizes the chemical reaction between a solid fuel charge and the resulting rising column of gas in the furnace. The three different materials used for the charge are ore, flux and coke. The ore consists of iron oxide about four inches in diameter. The flux is limestone that decomposes into calcium oxide and carbon dioxide. The lime reacts with impurities in the ore and floats them to the surface in the form of as lag. Coke, which is primarily carbon, is the ideal fuel for blast furnaces because it produces carbon monoxide gas, the main agent for reducing iron ore into iron metal. Primarily carbon is the ideal fuel for blast furnaces because it produces carbon monoxide gas, the main agent for reducing iron ore into iron metal.
The basic operation of the blast furnace is to reduce iron oxide to iron metal and to remove impurities from the metal. Reduced elements pass into the iron and oxidized elements dissolve into the slag. The metal that comes from the blast furnace is called pig iron and is used as a starting material for further purification processes.
Pig iron contains excessive amounts of elements that must be reduced before steel can be produced. Different types of furnaces, most notably the open hearth, electric and basic oxygen, are used to continue this refining process. Each furnace performs the task of removing or reducing elements such as carbon, silicon, phosphorus, sulfur and nitrogen by saturating the molten metal with oxygen and slag forming ingredients. The oxygen reduces elements by forming gases that are blown away and the slag attracts impurities as it separates from the molten metal.
Depending upon the type of slag that is used, refining furnaces are classed as either acid or basic. Large amounts of lime are contained in basic slags and high quantities of silica are present in acid slags. This differential between acid and basic slags is also present in welding electrodes for much of the same refining process occurs in the welding operation.
After passing through the refining furnace, the metal is poured into cast iron ingot molds. The ingot produced is a rather large square column of steel. At this point, the metal is saturated with oxygen. To avoid the formation of large gas pockets in the cast metal, a substantial portion of the oxygen must be removed. This process is known as de-oxidation, and it is accomplished through additives that tie up the oxygen either through gases or in slag. There are various degrees of oxidation, and the common ingots resulting from each are as follows:
Rimmed Steel – The making of rimmed steels involves the least de-oxidation. As the ingots solidify, a layer of nearly pure iron is formed on the walls and bottom of the mold, and practically all the carbon, phosphorus, and sulfur segregate to the central core. The oxygen forms carbon monoxide gas and it is trapped in the solidifying metal as blow holes that disappear in the hot rolling process. The chief advantage of rimmed steel is the excellent defect-free surface that can be produced with the aid of the pure iron skin. Most rimmed steels are low carbon steels containing less than .1% carbon.
Capped Steel – Capped steel regulates the amount of oxygen in the molten metal through the use of a heavy cap that is locked on top of the mold after the metal is allowed to reach a slight level of rimming. Capped steels contain a more uniform core composition than the rimmed steels. Capped steels are, therefore, used in applications that require excellent surfaces, a more homogenous composition, and better mechanical properties than rimmed steel.
Killed Steel – Unlike rimmed or capped steel, killed steel is made by completely removing or tying up the oxygen before the ingot solidifies to prevent the rimming action. This removal is accomplished by adding a ferro-silicon alloy that combines with oxygen to form a slag, leaving a dense and homogenous metal.
Semi-Killed Steel – Semi-Killed steel is a compromise between rimmed and killed steel. A small amount of deoxidizing agent, generally ferro-silicon or aluminum, is added. The amount is just sufficient to kill any rimming action, leaving some dissolved oxygen.
Vacuum Deoxidized Steel – The object of vacuum de-oxidation is to remove oxygen from the molten steel without adding an element that forms nonmetallic inclusions. This is done by increasing the carbon content of the steel and then subjecting the molten metal to vacuum pouring or steam degassing. The carbon reacts with the oxygen to form carbon monoxide, and as a result, the carbon and oxygen levels fall within specified limits. Because no deoxidizing elements that form solid oxides are used, the steel produced by this process is quite clean.
Classifications of Steel
The three commonly used classifications for steel are: carbon, low alloy, and high alloy. These are referred to as the “type” of steel.
Carbon Steel – Steel is basically an alloy of iron and carbon, and it attains its strength and hardness levels primarily through the addition of carbon. Carbon steels are classed into four groups, depending on their carbon levels.
Low Carbon Up to 0.15% carbon
Mild Carbon Steels .15% to 0.29% carbon
Medium Carbon Steels .30% to 0.59% carbon
High Carbon Steels .60% to 1.70% carbon
The largest tonnage of steel produced falls into the low and mild carbon steel groups. They are popular because of their relative strength and ease with which they can be welded.
Low Alloy Steel – Low alloy steel, as the name implies, contains small amounts of alloying elements that produce remarkable improvements in their properties. Alloying elements are added to improve strength and toughness, to decrease or increase the response to heat treatment, and to retard rusting and corrosion. Low alloy steel is generally defined as having a 1.5% to 5% total alloy content. Common alloying elements are manganese, silicon, chromium, nickel, molybdenum, and vanadium. Low alloy steels may contain as many as four or five of these alloys in varying amounts.
Low alloy steels have higher tensile and yield strengths than mild steel or carbon structural steel. Since they have high strength-to-weight ratios, they reduce dead weight in rail road cars, truck frames, heavy equipment, etc.
Ordinary carbon steels, that exhibit brittleness at low temperatures, are unreliable in critical applications. Therefore, low alloy steels with nickel additions are often used for low temperature situations.
Steels lose much of their strength at high temperatures. To provide for this loss of strength at elevated temperatures, small amounts of chromium or molybdenum are added.
High Alloy Steel – This group of expensive and specialized steels contain alloy levels in excess of 10%, giving them outstanding properties.
Austenitic manganese steel contains high carbon and manganese levels that give it two exceptional qualities, the ability to harden while undergoing cold work and great toughness. The term austenitic refers to the crystalline structure of these steels.
Stainless steels are high alloy steels that have the ability to resist corrosion. This characteristic is mainly due to the high chromium content, i.e., 10% or greater. Nickel is also used in substantial quantities in some stainless steels.
Tool steels are used for cutting and forming operations. They are high quality steels used in making tools, punches, forming dies, extruding dies, forgings and so forth. Depending upon their properties and usage, they are sometimes referred to as water hardening, shock resisting, oil hardening, air hardening, and hot work tool steel.
Because of the high levels of alloying elements, special care and practices are required when welding high alloy steels.
Many steel producers have developed steels that they market under a trade name such as Cor-Ten, HY-80, T-1, NA-XTRA, or SS-100, but usually a type of steel is referred to by its specification. A variety of technical, governmental and industrial associations issue specifications for the purpose of classifying materials by their chemical composition, properties or usage. The specification agencies most closely related to the steel industry are the American Iron and Steel Institute (AISI), Society of Automotive Engineers (SAE), American Society for Testing and Materials (ASTM), and the American Society of Mechanical Engineers (ASME).
The American Iron and Steel Institute (AISI) and the Society of Automobile Engineers (SAE) have collaborated in providing identical numerical designations for their specifications. The first two digits of a four digit index number refer to a series of steels classified by their composition or alloy combination. While the last two digits, which can change within the same series, give an approximate average of the carbon range. For example, the first two digits of a type 1010 or 1020 steel indicate a “10” series that has carbon as its main alloy. The last two digits indicate an approximate average content of.10% or .20% carbon, respectively. Likewise, the “41” of 4130 type steel refers to a group that has chromium and molybdenum as their main alloy combination with approximately .30% carbon content.
The AISI classifications for certain alloys, such as stainless steel, are somewhat different. They follow a three digit classification with the first digit designating the main alloy composition or series. The last two digits will change within a series, but are of an arbitrary nature being agreed upon by industry as a designation for certain compositions within the series. For example, the “3” in a 300 series of stainless steel indicates chromium and nickel as the main alloys, but a 308 stainless has a different overall composition than a347 type. The “4” of a 400 series indicates the main alloy as chromium, but there are different types such as 410, 420, 430, and so forth within the series.
The American Society for Testing and Materials (ASTM) is the largest organization of its kind in the world. It has compiled some 48 volumes of standards for materials, specifications, testing methods and recommended practices for a variety of materials ranging from textiles and plastics to concrete and metals.
Two ASTM designated steels commonly specified for construction are A36-77and A242-79. The prefix letter indicates the class of a material. In this case, the letter “A” indicates a ferrous metal, the class of widest interest in welding. The numbers 36 and 242 are index numbers. The 77 and 79 refer to the year that the standards for these steels were originally adopted or the date of their latest revision.
The ASTM designation may be further subdivided into Grades or Classes. Since many standards for ferrous metals are written to cover forms of steel (i.e., sheet, bar, plate, etc.) or particular products fabricated from steel (i.e., steel rail, pipe, chain, etc.), the user may select from a number of different types of steel under the same classification. The different types are than placed under grades or classes as a way of indicating the differences in such things as chemistries, properties, heat treatment, etc. An example of a full designation is A285-78 Grade A or A485-79 Class 70.
The American Society of Mechanical Engineers (ASME) maintains a widely used ASME Boiler and Pressure Vessel Code. The material specification as adopted by the ASME is identified with a prefix letter “S”, while the remainder is identical with ASTM with the exception that the date of adoption or revision by ASTM is not shown. Therefore, a common example of an ASME classification is SA 387 Grade 11, Class 1.
Crystalline Structure of Metals
When a liquid metal is cooled, its atoms will assemble into a regular crystal pattern and we say the liquid has solidified or crystallized. All metals solidify as a crystalline material. In a crystal the atoms or molecules are held in a fixed position and are not free to move about as are the molecules of a liquid or gas. This fixed position is called a crystal lattice. As the temperature of a crystal is raised, more thermal energy is absorbed by the atoms or molecules and their movement increases. As the distance between the atoms increases, the lattice breaks down and the crystal melts. If a lattice contains only one type of atom, as in pure iron, the conditions are the same at all points throughout the lattice, and the crystal melts at a single temperature (see Figure 1).
However, if the lattice contains two or more types of atoms, as in any alloy-steel, it may start to melt at one temperature but not be completely molten until it has been heated to a higher temperature (See Figure 2). This creates a situation where there is a combination of liquids and solids within a range of temperatures.
Each metal has a characteristic crystal structure that forms during solidification and often remains the permanent form of the material as long as it remains at room temperature. However, some metals may undergo an alteration in the crystalline form as the temperature is changed. This is known as phase transformation. For example, pure iron solidifies at 2795°F, the delta structure transforms into a structure referred to as gamma iron. Gamma iron is commonly known as austenite and is a nonmagnetic structure. At a temperature of 1670°F., the pure iron structure transforms back to the delta iron form, but at this temperature, the metal is known as alpha iron. These two phases are given different names to differentiate between the high temperature phase (delta) and the low temperature phase (alpha). The capability of the atoms of a material to transform into two or more crystalline structures at different temperatures is defined as allotropic. Steels and iron are allotropic metals.
Grains and Grain Boundaries – As the metal is cooled to its freezing point, a small group of atoms begin to assemble into crystalline form (refer to Figure 3). These small crystals scattered throughout the body of the liquid are oriented in all directions and as solidification continues, more crystals are formed from the surrounding liquid. Often, they take the form of dendrites, or a treelike structure. As crystallization continues, the crystals begin to touch one another, their free growth hampered, and the remaining liquid freezes to the adjacent crystals until solidification is complete. The solid is now composed of individual crystals that usually meet at different orientations. Where these crystals meet is called a grain boundary.
A number of conditions influence the initial grain size. It is important to know that cooling rate and temperature has an important influence on the newly solidified grain structure and grain size. To illustrate differences in grain formation, let’s look at the cooling phases in a weld.
Initial crystal formation begins at the coolest spot in the weld. That spot is at the point where the molten metal and the un-melted base metal meet. As the metal continues to solidify, you will note that the grains in the center are smaller and finer in texture than the grains at the outer boundaries of the weld deposit. This is explained by the fact that as the weld metal cools, the heat from the center of the weld deposit will dissipate into the base metal through the outer grains that solidified first. Consequently, the grains that solidified first were at high temperatures for a longer time while in the solid state and this is a situation that encourages grain growth. Grain size can have an effect on the soundness of the weld. The smaller grains are stronger and more ductile than the larger grains. If a crack occurs, the tendency is for it to start in the area where the grains are largest.
To summarize this section, it should be understood that all metals are composed of crystals of grains. The shape and characteristics of crystals are determined by the arrangement of their atoms. The atomic pattern of a single element can change its arrangement at different temperatures, and that this atomic pattern or micro structure determines the properties of the metals.
The temperature that metal is heated, the length of time it is held at that temperature, and the rate that it is cooled; all have an effect on a metal’s crystalline structure. This crystalline structure, commonly referred to as “microstructure,” determines the specific properties of metals. There are various ways of manipulating the microstructure, either at the steel mill or in the welding procedure. Some of the more common ways are as follows:
Preheat – Most metals are rather good conductors of heat. As a result, the heat in the weld area is rapidly dispersed through the whole weldment to all surfaces where it is radiated to the atmosphere causing comparatively rapid cooling. In some metals, this rapid cooling may contribute to the formation of microstructures in the weld zone that are detrimental. Preheating the weldment before it is welded is a method of slowing the cooling rate of the metal. The preheat temperature may vary from 150°F to 1000°F, but more commonly it is held in the 300°F to 400°F range. The thicker the weld metal, the more likely will it be necessary to preheat, because the heat will be conducted away from the weld zone more rapidly as the mass increases.
Stress Relieving – Metals expand when heated and contract when cooled. The amount of expansion is directly proportional to the amount of heat applied. In a weldment, the metal closest to the weld is subjected to the highest temperature, and as the distance from the weld zone increases, the maximum temperature reached decreases. This non-uniform heating causes non-uniform expansion and contraction and can cause distortion and internal stresses within the weldment. Depending on its composition and usage, the metal may not be able to resist these stresses and cracking or early failure of the part may occur. One way to minimize these stresses or to relieve them is by uniformly heating the structure after it has been welded. The metal is heated to temperatures just below the point where a microstructure change would occur and then it is cooled at a slow rate.
Hardening – The hardness of steel may be increased by heating it to 50°F to 100°F above the temperature that a microstructure change occurs, and then placing the metal in a liquid solution that rapidly cools it. This rapid cooling, known as “quenching,” locks in place microstructures known as “martensite” that contribute to a metal’s hardness characteristic. The quenching solutions used in this process are rated according to the speed that they cool the metal, i.e., Oil (fast), Water (faster), Salt Brine (fastest).
Tempering – After a metal is quenches, it is then usually tempered. Tempering is a process where the metal is reheated to somewhere below 1335°F, held at that temperature for a length of time, and then cooled to room temperature. Tempering reduces the brittleness that is characteristic in hardened steels, thereby producing a good balance between high strength and toughness. The term toughness, as it applies to metals, usually refers to resistance to brittle fracture or notch toughness under certain environmental conditions. More information on these properties will be covered later in this lesson and in subsequent lessons. Steels that respond to this type of treatment are known as “quenched and tempered steels.”
Annealing – A metal that is annealed is heated to a temperature 50° to 100° above where a microstructure change occurs, held at that temperature for a sufficient time for a uniform change to take place, and then cooled at a very slow rate, usually in a furnace. The principal reason for annealing is to soften steel and create a uniform fine grain structure. Welded parts are seldom annealed for the high temperatures would cause distortion.
Normalizing – The main difference between normalizing and annealing is the method of cooling. Normalized steel is heated to a temperature approximately 100° above where the microstructure transforms and then cooled in still air rather than in a furnace.
Heat Treatment Trade-Off – It must be noted that these various ways of controlling the heating and cooling of metals can produce a desired property, but sometimes at the expense of another desirable property. An example of this trade-off is evident in the fact that certain heat treatments can increase the strength or hardness of metal, but the same treatments will also make the metal less ductile or more brittle, and therefore, susceptible to welding problems.
Properties of Metals
The usefulness of a particular metal is determined by the climate and conditions in which it will be used. A metal that is stamped into an automobile fender must be softer and more pliable than armor plate that must withstand an explosive force, or the material used for an oil rig on the Alaska North Slope must perform in a quite different climate than a steam boiler. It becomes obvious that before a material is recommended for a specific use, the physical and mechanical properties of that metal and the weld metal designed to join it must be evaluated. Some of the more important properties of metals and the means of evaluation are as follows:
Tensile Strength – Tensile strength is one of the most important determining factors in selecting a metal, especially if it is to be a structural member, part of a machine, or part of a pressure vessel.220.127.116.11 The tensile test is performed as shown in Figure 4. The test specimen is machined to exact standard dimensions and clamped into the testing apparatus at both ends. The specimen is then pulled to the point of fracture and the data recorded.
The tensile strength test gives us 4 primary pieces of information: (1) Yield Strength, (2) Ultimate Tensile Strength, (3) Elongation, and (4)Reduction in Area.
Yield Strength – When a metal is placed in tension, it acts somewhat like a rubber band. When a load of limited magnitude is applied, the metal stretches, and when the load is released, the metal returns to its original shape. This is the elastic characteristic of metal and is represented by letter A in Figure 5. As a greater load is applied, the metal will reach a point where it will no longer return to its original shape but will continue to stretch. Yield strength is the point where the metal reaches the limit of its elastic characteristic and will no longer return to its original shape.
Ultimate Tensile Strength – Once a metal has exceeded its yield point, it will continue to stretch or deform, and if the load is suddenly released, the metal will not return to its original shape, but will remain in its elongated form. This is called the plastic region of the metal and is represented by the letter B in Figure 5. As this plastic deformation increases, the metal strains against further elongation, and an increased load must be applied to stretch the metal. As the load is increased, the metal will finally reach a point where it no longer resists and any further load applied will rapidly cause the metal to break. That point at which the metal has withstood or resisted the maximum applied load is its ultimate tensile strength. This information is usually recorded in pounds per square inch (psi).
Percentage of Elongation – Before a tensile specimen is placed in the tensile tester, two marks at a measured distance are placed on the opposing ends of the circular shaft. After the specimen is fractured, the distance between the marks is measured and recorded as a percentage of the original distance. See Figure 5. This is the percentage of elongation and it gives an indication of the ductility of the metal at room temperature.
Reduction of Area – A tensile specimen is machined to exact dimensions. The area of its midpoint cross-section is a known figure. As the specimen is loaded to the point of fracture, the area where it breaks is reduced in size. See Figure 5. This reduced area is calculated and recorded as a percentage of the original cross-sectional area. This information reflects the relative ductility or brittleness of the metal.
Charpy Impacts – Metal that is normally strong and ductile at room temperature may become very brittle at much lower temperatures, and thus, is susceptible to fracture if a sharp abrupt load is applied to it. An impact tester measures the degree of susceptibility to what is called brittle fracture.
The impact specimen is machined to exact dimensions (Figure 6) and then notched on one side. Quite often, the notch is in the form of a “V” and the test in this case is referred to as a Charpy V-Notch Impact Test. The specimen is cooled to a predetermined temperature and then placed in a stationary clamp at the base of the testing machine. The specimen is in the direct path of a weighted hammer attached to a pendulum (Figure 6).
The hammer is released from a fixed height and the energy required to fracture the specimen is recorded in ft-lbs. A specimen that is cooled to -60°F and absorbs 40 ft-lbs of energy is more ductile, and therefore, more suitable for low temperature service than a specimen that withstands only 10 ft-lbs at the same temperature. The specimen that withstood 40 ft-lbs energy is said to have better toughness or notch toughness.
Fatigue Strength – A metal will withstand a load less than its ultimate tensile strength but may break if that load is removed and then reapplied several times. For ex-ample, if a thin wire is bent once, but if it is bent back and forth repeatedly, it will eventually fracture and it is said to have exceeded its fatigue strength. A common test for this strength is to place a specimen in a machine that repeatedly applies the same load first intension and then in compression. The fatigue strength is calculated from the number of cycles the metal withstands before the point of failure is reached.
Creep Strength – If a load below a metal’s tensile strength is applied at room temperature (72°F), it will cause some initial elongation, but there will be no further measurable elongation if the load is kept at a constant level. If that same load were applied to a metal heated to a high temperature, the situation would change. Although the load is held at a constant level, the metal will gradually continue to elongate. This characteristic is called creep. Eventually, the material may rupture depending on the temperature of the metal, the degree of load applied and the length of time that it is applied. All three of these factors determine a metal’s ability to resist creep, and therefore, its creep strength.
Oxidation Resistance – The atoms of metal have a tendency to unite with oxy-gen in the air to form oxide compounds, the most visible being rust and scale. In some metals, these oxides will adhere very tightly to the skin of the metal and effectively seal it from further oxidation as is evident in stainless steel. These materials have high oxidation resistance. In other metals, the bond is very loose, creating a situation where the oxides will flake off, and the metal gradually deteriorates as the time of exposure is extended.
Hardness Test – The resistance of a metal to indentation is a measure of its hardness and an indication of the material’s strength. To test for hardness, a fixed load forces an indenter into the test material (Figure 7). The depth of the penetration or the size of the impression is measured. The measurement is converted into a hardness number through the use of a variety of established tables. The most common tables are the Brinell, Vickers, Knoop and Rockwell. The Rockwell is further divided into different scales, and depending on the material being tested, the shape of the indenter and the load applied, the conversion tables may differ. For example, a material listed as having a hardness of Rb or Rc means its hardness has been determined from the Rockwell “B” scale or the Rockwell “C” scale.
Coefficient of Expansion – All metals expand when heated and contract when cooled. This dimensional change is related to the crystalline structure and will vary with different materials. The different expansion and contraction rates are expressed numerically by a coefficient of thermal expansion. When two different metals are heated to the same temperature and cooled at the same rate, the one with the higher numerical coefficient will expand and contract more than the one with the lesser coefficient.
Thermal Conductivity – Some metals will absorb and transmit heat more readily than others. They are categorized as having high thermal conductivity. This characteristic contributes to the fact that some metals will melt or undergo transformations at much lower temperatures than others.
Effects of the Alloying Elements
Alloying is the process of adding a metal or a nonmetal to pure metals such as copper, aluminum or iron. From the time it was discovered that the properties of pure metals could be improved by adding other elements, alloy steel has increased by popularity. In fact, metals that are welded are rarely in their pure state. The major properties that can be improved by adding small amounts of alloying elements are hardness, tensile strength and ductility and corrosion resistance. Common alloying elements and their effect on the properties of metals are as follows:
Carbon – Carbon is the most effective, most widely used and lowest in cost alloying element available for increasing the hardness and strength of metal. An alloy containing up to 1.7% carbon in combination with iron is known as steel, whereas the combination above 1.7% carbon is known as cast iron. Although carbon is a desirable alloying element, high levels of it can cause problems; therefore, special care is required when welding high carbon steels and cast iron.
Sulphur – Sulphur is normally an undesirable element in steel because it causes brittleness. It may be deliberately added to improve the machinability of the steel. The sulphur causes the machine chips to break rather than form long curls and clog the machine. Normally, every effort is made to reduce the sulphur content to the lowest possible level because it can create welding difficulties.
Manganese – Manganese in contents up to 1% is usually present in all low alloy steels as a deoxidizer and desulphurizer. That is to say, it readily combines with oxygen and sulphur to help negate the undesirable effect these elements have when in their natural state. Manganese also increases the tensile strength and hardenability of steel.
Chromium – Chromium, in combination with carbon, is a powerful hardening alloying element. In addition to its hardening properties, chromium increases corrosion resistance and the strength of steel at high temperatures. Chromium is the primary alloying element in stainless steel.
Nickel – The greatest single property of steel that is improved by the presence of nickel is its ductility or notch toughness. In this respect, it is the most effective of all alloying elements in improving steel’s resistance to impact at low temperatures. Electrodes with high nickel content are used to weld cast iron materials. Nickel is also used in combination with chromium to form a group known as austenitic stainless steel.
Molybdenum – Molybdenum strongly increases the depth of the hardening characteristic of steel. It is quite often used in combination with chromium to improve the strength of the steel at high temperatures. This group of steels is usually referred to as chrome-moly steels.
Silicon – Silicon is usually contained in steel as a deoxidizer. Silicon will add strength to steel but excessive amounts can reduce the ductility. Additional amounts of silicon are sometimes added to welding electrodes to increase the fluid flow of weld metal.
Phosphorus – Phosphorus is considered a harmful residual element in steel because it greatly reduces ductility and toughness. Efforts are made to reduce it to its very lowest levels; however, phosphorus is added in very small amounts to some steels to increase strength.
Aluminum – Aluminum is primarily used as a deoxidizer in steel. It may also be used in very small amounts to control the size of the grains.
Copper – Copper contributes greatly to the corrosion resistance of carbon steel by retarding the rate of rusting at room temperature, but high levels of copper can cause welding difficulties.
Columbium – Columbium is used in austenitic stainless steel to act as a stabilizer. Since the carbon in the stainless steel decreases the corrosion resistance, a means of making carbon ineffective must be found. Columbium has a greater affinity for carbon than chromium, leaving the chromium free for corrosion protection.
Tungsten – Tungsten is used in steel to given strength at high temperatures. Tungsten also joins with carbon to form carbides that are exceptionally hard, and therefore have exceptional resistance to wear.
Vanadium – Vanadium helps keep steel in the desirable fine grain condition afterheat treatment. It also helps increase the depth of hardening and resists softening of the steel during tempering treatments.
Nitrogen – Usually, efforts are made to eliminate hydrogen, oxygen and nitrogen from steel because their presence can cause brittleness. Nitrogen has the ability to form austenitic structures; therefore, it is sometimes added to austenitic stainless steel to reduce the amount of nickel needed, and therefore, the production costs of that steel.
Alloying Elements Summary – It should be understood that the addition of elements to a pure metal may influence the crystalline form of the resultant alloy. If a pure metal has allotropic characteristics (the ability of a metal to change its crystal structure) at a specific temperature, then that characteristic will occur over a range of temperatures with the alloyed metal. The range in which the change takes place may be wide or narrow, depending on the alloys and the quantities in which they are added. The alloying element may also effect the crystalline changes by either suppressing the appearance of certain crystalline forms or even by creating entirely new forms. All these transformations induced by alloying elements are dependent on heat input and cooling rates. These factors are closely controlled at the steel mill, but since the welding operation involves a nonuniform heating and cooling of metal, special care is often needed in the welding of low and high alloy steel.