January 11, 2011
Welders may recognize that when porosity is present, the cause isn’t always the shielding gas, welding filler, or operator error. It can be caused by the steel production method, which can introduce porosity and other flaws.
Metal production processes introduce flaws (dimensional and integral discontinuities) and cleaning requirements that affect welding.
Most metals actually are mined as minerals—metal combined with another element. Commonly used metals—not the exotic metals—are combined with nitrogen, oxygen, or sulfur. Those combined with oxygen are called oxides. Metals combined with sulfur are called sulfides, and those containing nitrogen are called nitrides. (Some stainless steels are treated with nitrogen to harden the surface.)
For these metals to be made into useful materials, separation or reduction must occur (Figure 1). These processes are accomplished by heating and melting the minerals.
The reduction process simply reduces the amount of nitrogen, oxygen, and sulfur combined with the metal within a mineral. There is little reason to do an in-depth study relating to this process, except to say that excessive amounts of nitrogen, oxygen, and sulfur are undesirable in metals to be welded. At this time, not all sulfur, nitrogen, and oxygen can be removed. Emerging vacuum degassing technology likely will make almost total removal possible within this decade.
Steel used to produce welding filler metal usually is referred to as killed steel. Certain deoxidizers are added to reduce the oxygen, the most common being silicon (silicon-killed).
Aluminum powder and copper also are used to produce killed steel. The copper coatings on the ER electrodes and rods serve as protection from the atmosphere (to prevent rust on the wire) and as a deoxidizer in the weld. Some copper dissolves into the actual weld. Copper also enhances the conductivity at the contact tip. However, this small amount has little effect on the weld properties.
Semikilled steel is partially deoxidized, mainly through the use of aluminum or ferrosilicon. This steel typically is used in structural fabrications.
The most common semikilled steel is ASTM A36, which is produced in plate and shapes. This is a highly ductile material, but it often contains enough sulfur and manganese to produce manganese sulfides, which can create lamellar inclusions. These inclusions are not necessarily detrimental if their orientation is in the linear direction of tension or compression. However, if their orientation is such that there is tension in the through-thickness direction of the material, it presents a severe problem and often causes lamination or delamination. Delamination is the total separation of the material, usually at or near the center and edge of the material. These inclusions also may cause what welders refer to as “blowouts” as the weld crosses the inclusion.
Specifications frequently state that no rimmed or capped steels may be used in certain fabrications because of the porosity and other flaws often found in these materials. During production, deoxidizers are added in the crucible when the metal is ready to pour and consequently do not have enough time to react before the metal begins to solidify. Since the outer portion of the metal transforms from liquid to solid first (dendritic action), the gases cannot escape to the surface. This action causes discontinuities commonly referred to as pipes. Pipes are totally unacceptable in critical applications and cannot be tolerated.
Many U.S. steel mills now are using a vacuum degassing process previously mentioned rather than, or in addition to, using the deoxidizing elements such as aluminum and silicon.
Welders may recognize that when porosity is present, the cause isn’t always the shielding gas, welding filler, or operator error. It can be caused by the steel production method.
Hot Rolling. For several years steel has been produced by the hot-rolled method in which the molten metal flows through a set of high-carbon, heat-resistant rolls. This process is duplicated over and over by melting ingots and bars until the desired thickness or shape is achieved.
The rolling process in a sense “welds” the porosity and voids together. This process also reduces the grain size and thereby increases the metal’s ductility and integrity. However, a mill scale forms during the process that must be removed before welding. Hot-rolled shapes, such as angles and beams, have a much heavier scale than hot-rolled plates or sheets.
Oxides form on the outside of the material when it is exposed to air. In the case of steel, this oxide is in the form of rust. In some materials, such as aluminum, chromium-bearing material (i.e., stainless steels), and copper, the oxide formation actually enhances the material’s ability to resist corrosion and wear. All of these oxides must be removed before welding. The old saying “burn through the stuff” is not the way to go.
Cold Rolling. If a smooth surface is desired, the material may be cold-rolled. This process uses a much lower temperature as the material is passing through the rolls. The cold working not only achieves a smoother finish, but also increases the material’s strength.
Shafts that are to be used in the “as rolled” condition often are cold-rolled. Cold-rolled material also is used in the automotive industry for parts such as starter housings, power steering pump housings, drive axles, and shafts. Attention must be paid to the amount of hardness that is induced by cold working, but it usually is not extremely detrimental.
Extruding. Several other methods are used to achieve shapes such as beams, angles, channels, T bars, and round bars. One is called extrusion. This is a force-fed method in which the hot metal (around 1,800 degrees F) is forced through an extruding die that is internally shaped to form the external shape of the final product. Today’s technology allows these products to be made and used with very little postproduction cleaning. The mill scale is partially removed as the product proceeds through the dies. This process is not limited to only those shapes described previously.
Continuous Casting. Among the most successful recent technology for producing metal is the continuous casting process (C casting) This method uses equipment similar to that used in hot and cold rolling, but instead of repeating the rolling process in several series, the metal is water- or air-cooled, either as it passes through, or as it exits the rolls. Material produced with this process exhibits exceptionally true dimensions and very few undesirable inclusions. The elimination or minimization of inclusions greatly enhances the process of welding splices.
Metal Pressing. Pressed metal technology (Figure 2) is used in the production of pressure vessel heads. These heads are made to order and come in various sizes and shapes, including elliptical, dished, hemispherical, flanged, and double-flanged. Elliptical, dished, and hemispherical heads are the most common for higher pressures. Producers have discovered methods for minimizing or eliminating thinning of the material in the radius area. In most cases the heads are ordered with the proper weld bevels and other credentials that comply with the ASME or other code requirements.
One of the oldest methods of producing metal is foundry casting. In this process, alloying material is added either before or after the steel is melted. The alloying agents (particularly carbon) differentiate cast iron from cast steel. With casting, the same parts can be reproduced over and over using a pattern and a mold. Odd shapes and multiple alloys also can be produced with this process.
High-chromium nickel castings are excellent for heat resistance. The high-carbon, high-chromium nickel materials are not easily fabricated; casting is the best production method for these materials.
Welders generally do not enjoy working with cast iron because of its varying chemical compositions. Welding this material is difficult but not always impossible. Pre-established techniques and matching welding materials are not readily available in most cases.
The number of foundries has diminished drastically in the past 20 years. EPA regulations have made the control of emissions from these foundries very expensive. My employer closed its foundry in 1984 mostly because complying with the clean air standards would have required rebuilding the plant.
The American Foundrymen’s Society (ASF) is an excellent source for information about cast materials (AFS). Another good source is the Steel Founders’ Society of America (SFSA).
Forging (Figure 3) is the oldest method of refining materials. It also is likely the oldest method used for joining materials. My family name, Smith, is derived from this process, which was used even in Biblical history. “They shall beat their swords into plowshares” was a form of forging. My grandfather used a forge and hammer to arch leaf springs for automobiles and horse-drawn wagons. This was accomplished by hammering compressive stresses inside the curvature of the leaf. The operator of a forge is still referred to as a smith. The “blacksmith” utilizes the “man and hammer” method even today. The most common profession for the use of smith forging today is for horse shoeing. Running a close second is the arts and crafts smithy. The old hand-cranked coal-fired forge draws quite a crowd at the arts and crafts fairs.
Industrial Forging. The industrial forging process uses several different types of energy to power the hammer. In the 1950s and earlier, most hammers were steam-powered. The shape of the hammer was exacted to the die (sometimes referred to as an anvil). Half-round dies were used for round shaft material and flat dies for square or rectangular pieces. The flat dies required rotating the part 90 degrees after each blow of the hammer. The hammer and die method forced the grain to tighten to make a stronger, more uniform billet. This method was sometimes referred to as drawing out. The smaller grain not only increased the strength, but also minimized the discontinuities within the metal. Advertisements often referred to the material as “purely pure.”
Compression (Upset) Forging. Another type of forging is compression or upset forging, which utilizes a continuous pressure on the ends of the forging, thereby increasing the diameter or thickness of the forged piece. This process is used commonly to produce bolts, pins, and other fasteners. Forged material is very clean, nearly inclusion-free, and very suitable for welding.
Other metals are produced by methods similar to those used in steel production. To learn more about the production of those materials, see Welding Metallurgy by George Linnert and Basic Metallurgy by Donald V. Brown. These are both excellent books and may be used for texts or reference. Linnert’s book addresses more the welding aspect of metallurgy, while Brown looks more deeply into the production and usage of the metal (especially aluminum).
More information for aluminum materials may be found by contacting the Aluminum Association. The Nickel Institute is undoubtedly the superior source for information concerning nickel materials. For production and welding of titanium materials, see the International Titanium Association (ITA). ASM Intl. has numerous books containing this type of information.