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Many contractors are tailoring their welding methods to the construction of these types of plants, which tend to be modular in design and involve mostly large-bore pipe welding of carbon steels and low chrome alloys. Some of these power plant projects can span two to three years, with jobsite welding needs for about half of that time. A single job can require 30 or 40 — or even more than — full-time welding operators.

Finding skilled welding operators to complete the work in a timely manner and meet all the necessary requirements is a significant issue for power generation contractors. The welding industry faces a growing shortage of skilled welding operators, due both to a lack of incoming welding operators and the aging population of the current welding workforce.

This shortage is impacting many industries and markets, including power generation. The welding operator shortage also impacts other critical issues in power generation: Schedules are of critical importance for any power plant job, since there may be a requirement from the utility company or plant owner to have the plant operational by a certain date, according to Justin Morse, district welding engineer, Kiewit Power Constructors Co.

If deadlines are missed, power must be bought or may have a very high operational cost, which results in lost revenue borne by the utilities. These processes are generally easier to learn and use, making it easier to quickly train and qualify skilled welding operators for the requirements of power generation welding jobs.

In a modified short-circuit MIG process, the welding system anticipates and controls the short circuit, then reduces the welding current to create a consistent metal transfer.

Precisely controlled metal transfer provides uniform droplet deposition, making it easier for the welding operator to control the puddle. Advanced welding processes such as pulsed MIG or modified short-circuit MIG are also more forgiving to variations in stickout and result in a calm stable arc, which is easier for operators to control. These processes also provide increased productivity and efficiency by way of travel speeds that are three to four times those of TIG or stick welding.

On some alloys, the weld procedures allow for no back purge, which results in savings of operator time and material costs and helps improve productivity. P91 also resists corrosion better than steel alloys used previously in these applications. Because of these advantages, P91 is used extensively in power generation applications, such as for high pressure steam lines.

Due to the critical nature of these applications, any defects in the weld can result in cracking and eventual part failure. Failures in high-pressure steam pipe can be catastrophic, causing ruptures, tank or valve explosions, or other serious incidents that can result in fatal injuries.

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As a result, codes and procedures for P91 applications are stringent. Too much or too little heat can lead to issues with cracking in the heat affected zone HAZ and subsequent premature failure of the weld.

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The HAZ represents a region of potential weakness in P91 welding — and a major factor in the quality and service life of the weld. Preheating helps drive off moisture and reduce hydrogen, and it also reduces the thermal gradient between the base material and the weld puddle to improve weldability and keep the weld pool from cooling too quickly. Typically, P91 must be preheated in a temperature range from to degrees Fahrenheit, though the appropriate heating level depends on the qualified weld procedure used for construction and fabrication of the piping system.

Too cool and cracking may occur; too hot and the material can lose its strength and toughness. In addition, keep in mind that the more heat applied and the longer heat is applied, the larger the HAZ, which can increase opportunities for cracking.

The heat of the welding process makes the weld deposit and neighboring HAZ brittle when welding P Post-weld heat treatment restores toughness to the weld deposit and the HAZ. Because precision and stability in temperature control are extremely important when welding P91 pipe, induction heating is a solution that offers better control and more uniform heating of the part.

Induction, a form of electric preheat, does not rely on a heating element or flame to transfer heat. Instead, an alternating current passes through the device, creating a magnetic field around it. As the magnetic field passes through the conductive workpiece, it creates eddy currents within the part. Resistance to changing electrical eddy currents generates heat in the part. The part becomes its own heating element, heating from within, which makes induction very efficient since little heat is lost in the process.

Working with Type L Wrought stainless steels of the American Iron and Steel Institute AISI series are essentially austenitic at room temperature, consisting of grains in which the crystal lattice is of the face centered cubic fcc configuration of iron.

Welds on these materials, which are structurally equivalent to cast material, generally contain some ferrite grains in which the crystal structure is in the body centered cubic bcc configuration.

Advances in steel refining technology and analysis methods over the past decade have made it possible to precisely control the chemical composition of series materials. Since small changes in the percentage of alloying elements and trace elements can markedly affect performance, weldability, machinability, corrosion resistance, and surface finish, several subgroup specifications have been developed within the AISI specification.

In the temperature range of to 1, degrees Fahrenheit, carbon dissolved in the austenite grains comes out of solution and draws the chromium into the grain boundaries. The loss of chromium leaves the metal grains vulnerable to corrosion.

A reduced-carbon or "L" or ELC, extra-low carbon version of has been designed with a 0. The reduced amount of carbon decreases the loss of chromium from the grain boundaries in the sensitizing temperature range, which may occur either during welding or in service. Both orbital welding, which controls the heat input during welding, and the "L" grade of have lessened the concern over carbide precipitation.

Type L is referred to in the DIN standard as alloys 1.

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Type L, designated as S in the UNS numbering system, is favored by the high-purity industries and is relatively easy to machine and weld. The requirement for a precision fit between the components being welded is more stringent for orbital welds, which must have consistent uniform end preparations to achieve consistent high-quality welds. As long as the cutting blade on the facing equipment remains sharp, the chip from a L tube end comes off in a long spiral ribbon and the tube end remains square, with little or no burr.

Tubing for the high-purity industries is typically welded autogenously with no filler metal. The welding process subjects materials to a thermal cycle that affects the distribution of elements in the surface film and element segregation within the grains.

Although these changes affect the base metal's corrosion resistance and mechanical properties, if welding is done without excessive heat and with a purge to prevent excessive heat tint due to oxidation, the changes in corrosion resistance of L after orbital welding are minimal. It is less expensive and simpler to install a piping system by autogenous orbital welding than by orbital welding with the addition of filler to the weld required when welding higher-alloy material.

Thus, a material such as L, which can be welded without the addition of filler material while retaining its mechanical properties and corrosion resistance, offers an advantage. A variety of fittings and other piping system components is available with end preparations suitable for orbital welding and mechanically or electropolished interior surface finishes suitable for either semiconductor or biopharmaceutical applications. While stainless steel, especially electropolished Type L, is expensive, it is less costly than some other corrosion-resistant alloys.

If proper fabrication and welding procedures are followed, piping systems made from L tubing should have a long, productive service life.

Research has shown that elements such as sulfur and oxygen, which cause the temperature coefficient of surface tension to be positive, result in a weld puddle in which heat is transferred from the perimeter inward and downward with good penetration of the weld bead.

Removing sulfur and oxygen, either by refining or by the presence of elements that combine with sulfur or oxygen, such as aluminum, has an opposite effect on penetration.

In the latter case, the temperature coefficient of surface tension becomes negative, producing a wide, shallow weld with poor penetration and a tendency toward concavity see Figure 2. Other elements, including manganese and silicon, have slight effects on penetration, but sulfur has by far the greatest effect. However, sulfur in stainless steel combines with manganese to form nonmetallic inclusions called manganese sulfide MnS "stringers.

The tiny pits show up on SEMs scanning electron micrographs used to screen tubing samples for surface finish qualification.

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Since pitted surfaces are undesirable for high-purity applications and are typically the first places to show evidence of corrosion, tubing manufacturers and distributors have rallied to drive down the sulfur content of L tubing. The supplement S2 limits the sulfur content of this grade to 0. These values allow for ease of welding with lower MnS inclusions than would be found at the higher sulfur values of L.

With moderate to high sulfur content, type L is easier to machine than the low-sulfur materials, so it is favored by some fitting manufacturers. Thus, engineers, contractors, and welding personnel must take care in ordering tubing and fittings and record and track material heat numbers during fabrication to avoid costly problems. The base metal has a finer, more uniform grain structure than conventional type L, and orbital welds on this material have a much smoother appearance. Stainless steel produced by the electron beam refining EBR process has been used experimentally for special ultra-high-purity UHP semiconductor applications.

This EBR process uses entirely virgin materials in the melt, producing an unusually clean material. Manganese and other trace elements are reduced to very low levels, resulting in reduced stringers and improved corrosion resistance. This material also has less of a tendency to discolor from oxidation during welding.

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The blue "halo" that typically appears on either side of a weld on some heats of material does not appear on properly purged EBR material.

These projections might result in some particulation into UHP gas lines see Figure 3. One recurring problem with conventional type is the tendency for some heats to form slag islands or weld dross on the OD or ID weld bead. A change in shielding gas or weld parameters may reduce this problem, but it is difficult to eliminate entirely. Slag typically contains silicon and compounds of calcium and aluminum, which are added to standard melts to remove impurities.

The EBR melt is very low in impurities, so additions to remove them are unnecessary, and the slagging problem is nonexistent. Stainless steel's lack of consistent weldability from heat to heat has kept orbital GTAW from becoming a fully automatic process. Weld programs for each tubing diameter and wall thickness are entered into the power supply memory. A weld program or schedule will produce consistent uniform welds on a particular batch of tubing, but if tubing of a different heat number is introduced, some adjustment of amperage may be required, and parameter verification through test coupons is advisable.

Controlling the chemical composition of L materials with advanced refining technologies might eventually minimize heat-to-heat variations in penetration and allow orbital GTAW of tube to become a nearly automatic process.

This will save the production time currently spent on optimizing weld programs for individual material heats. Welding Type for Retention of Corrosion Resistance Welding and fabricating tubing may result in a loss of corrosion resistance relative to the unwelded base metal. The HAZ of welds has been implicated in the formation of rouge, a rust-like film containing the products of corrosion, in pharmaceutical water systems.

Contamination of stainless steel tubing, particularly with carbon, carbon steel, or chlorides, can severely affect corrosion resistance.