Welding Ultra-High-Strength Steels

The term high-strength steel is often applied to all steels other than mild low-carbon steels. The steels which have yield strength over 560 MPa are sometimes called the ultra-high-strength steels or super alloys.

The groups of steels that fall into this category are:

• Medium-carbon low-alloy hardenable steels
• Medium-alloy hardenable or tool and die steels
• High-alloy hardenable steels
• High-nickel maraging steels
• Martensitic stainless steels
• Semi austenitic precipitation-hardenable stainless steels

Medium-Carbon Low-Alloy Hardenable Steels

The best-known steels in this class are AISI 4130 and AISI 4140 steels. Also in this class are the higher-strength AISI 4340 steel and the AMS 6434 steel. These steels obtain their high strength by heat treatment to a full martensitic microstructure, which is tempered to improve ductility and toughness.

Tempering temperatures greatly affect the strength levels of these steels. The carbon is in the medium range and as low as possible but sufficient to give the required strength. Impurities are kept to an absolute minimum because of high-quality melting and refining methods.

These steels are available as sheets, bars, tubing, and light plate. The steels in this group can be mechanically cut or flame cut. However, when they are flame cut they must be preheated to 316°C. Flame-cut parts should be annealed before additional operations in order to reduce the hardness of the flame-cut edges.

These steels are suitable for welding only when they are in the annealed or normalized condition. After welding, they have to be heat treated to obtain the desired strength. The gas tungsten arc, the gas metal arc, the shielded metal arc, and the gas welding process are all used for welding these steels. The composition of the filler metal is designed to produce a weld deposit that responds to a heat treatment in approximately the same manner as the base metal.

In order to avoid brittleness and the possibility of cracks during welding, relatively high preheat and interpass temperatures are used. Preheating is in the order of 316°C. Complex weldments are heat treated immediately after welding.

Aircraft engine parts, aircraft tubular frames, and racing car frames are made from AISI 4130 tubular sections. These types of structures are normally not heat treated after welding.

Medium-Alloy Hardenable Steels

These steels are used largely in the aircraft industry for ultra-high-strength structural applications. They have carbon in the low to medium range and possess good fracture toughness at high-strength levels. In addition, they are air hardened, which reduces the distortion that is encountered with more drastic quenching methods. Some of the steels in this group are known as hot work die steels and another grade has become known as 5Cr-Mo-V aircraft quality steel. These steels are available as forging billets, bars, sheet, strip, and plate.

There is another type of steel in this general class which is a medium-alloy quenched and tempered steel known as high-yield or HY 130/150. This type of steel is used for submarines, aerospace applications, and pressure vessels, and is normally available as plate. This steel has good notch toughness properties at 0°C and below. These types of steels have much lower carbon than the grades mentioned previously.

When flame cutting or welding the aircraft quality steels, preheating is absolutely necessary since the steels are air hardening. A preheating on 316°C is used before flame cutting and then annealed immediately after the flame-cutting operation. This will avoid a brittle layer at the flame-cut edge, which is susceptible to cracking.

These types of steel should only be welded in the annealed condition. The steel should be preheated to 316°C and this temperature must be maintained throughout the welding operation. After welding, the work must be cooled slowly. This can be done by post heating, or by furnace cooling. The weldment is then stress relieved at 704°C and air cooled to obtain a fully tempered microstructure suitable for additional operations. It is usually annealed, after all welding is done, prior to final heat treatment. The filler metal should be of the same com-position as the base metal. The gas tungsten arc and gas metal arc processes are most widely used. However, shielded metal arc welding, plasma arc, and electron beam welding processes can be used.

The medium-alloy quenched and tempered high-yield strength steels are usually welded with the shielded metal arc, gas metal arc, or the submerged arc welding process. The filler metal must provide deposited metal of a strength level equal to the base material. In all cases, a low-hydrogen or no-hydrogen process is required.

For shielded metal arc welding the low-hydrogen electrodes of the E-13018 type are recommended. Electrodes must be properly stored. In the case of the other processes, precautions should be taken to make sure that the gas is dry and that the submerged arc flux is dry. By employing the proper heat input-heat output procedure yield strength and toughness are maintained. Preheating should be at least at 38°C for thinner materials. For heavier materials preheating temperature has to be higher.

The heat input should be such that the adjacent base metal does not become overheated while the heat output is sufficient to maintain the proper microstructure in the heat-affected zone. There may be some softening in the intermixing zone. The properties of welded joints that are properly made will be in the same order as the base metal. Subsequent heat-treating is usually not required or desired.

High-Alloy Hardenable Steels

The steels in this group develop high strength by standard hardening and tempering heat treatments. The steels possess extremely high strength in the range of 1240 MPa yield and have a high degree of toughness. This is obtained with a minimum carbon content usually in the range of 0.20%; however, these steels contain relatively high amounts of nickel and cobalt, and they are sometimes called the 9 Ni-4 Co steels. These steels also contain small amounts of other alloying elements.

They are normally welded in the quenched and tempered condition by the gas tungsten arc welding process. No post-heat treatment is required. The filler metal must match the analysis of the base metal.

High-Nickel Maraging Steels

This type of steel has relatively high nickel, and low carbon content. It obtains its high strength from a special heat treatment called maraging. These steels possess an extraordinary combination of ultra-high-strength and fracture toughness and at the same time are formable, weldable, and easy to heat treat. There are three basic types: the steels with 18% nickel, 20% nickel, and 25% nickel. These steels are available in sheet, forging billets, bars, strip, and plate. Some are available as tubing.

The extra special properties of these steels are obtained by heating the steel to 482°C and allowing it to cool to room temperature. During this heat treatment all of the austenite transforms to martensite. The heating time at the 482°C temperature is extremely important and usually is in the range of three hours. The steels derive their strength while aging at this temperature in the martensitic condition and for this reason are known as maraging steels.

These steels are supplied in the soft or annealed condition. They can be cold worked in this condition and can be flame cut or plasma arc cut. Plasma arc cutting is preferred.

These steels are usually welded by the gas tungsten arc or the gas metal arc welding process. The shielded metal arc and submerged arc process can also be used with special electrode-flux combinations. The filler metal should have the same composition as the base metal. In addition, the filler metal must be of high purity with low carbon. Preheat or postheat is not required; however, the welding must be followed by the maraging heat treatment which produces weld joints of an extremely high strength.

Martensitic Stainless Steels

These steels are of the straight chromium type, such as AISI 420. They contain 12-14% chromium and up to 0.35% carbon. This composition combines stainlessness with high strength. Numerous variations of this basic composition are available, all of which are in the martensitic classification.

This type of steel has been used for compressor and turbine blades of jet engines and for other applications in which moderate corrosion resistance and high strength are required. The strength level of these steels is obtained by a quenching and tempering heat treatment. They can be obtained as sheet, strip, tubing, and plate. The compositions are also used for castings. These steels can be heat treated to strengths as high as 1750 MPa yield strength.

These stainless steels can be flame cut by the powder cutting system normally used for flame cutting stainless steels. They can also be cut with the oxy-arc process. Flame cutting should be done with the steel in the annealed condition. Most grades should be preheated to 316°C because they are air hardenable. They should be annealed after cutting to restore softness and ductility. These materials can also be cold worked in the annealed condition.

The martensitic stainless steels can be welded in the annealed or fully hardened condition, usually without preheat or postheat. The gas tungsten arc welding process is normally used. The filler metal must be of the same analysis as the base metal. Following welding the weldment should be annealed and then heat treated to the desired strength level.

Semiaustenitic Precipitation-Hardenable Stainless Steels

The steels in this group are chrome-nickel steels that are ductile in the annealed condition but can be hardened to high strength by proper heat treatment. In the annealed condition the steels are austenitic and can be readily cold worked. By special heat treatment the austenite is transformed to martensite and later a precipitant is formed in the martensite. The outstanding extra high strength is obtained by a combination of these two hardening processes.

The term semi austenitic type was given these steels to distinguish them from normal stainless steels. They are also called precipitation hardening steels or PH steels. The heat treatment for these steels is based on heating the annealed material to a temperature between 927°C and 954°C, followed by a tempering or aging treatment in the range of 454-593°C. These steels are available as billets, sheets, tubing, and plates.

These steels are normally not flame cut. Welding is performed using the gas tungsten arc or the gas metal arc welding process. The shielded metal arc welding process is rarely used. The filler metal should have the same composition as the base metal. No preheat or postheat is required if the parts are welded in the annealed condition. After welding, the steel has to be heat-treated to develop optimum strength levels.

However, there is a loss of joint strength due to heating of the heat-affected zone above the aging temperature. In view of this, it is not possible to produce a 100% efficient joint. Extra reinforcing must be utilized to develop full-strength joints. These steels are also brazed using nickel alloy filler metal.

When welding on any of these high-strength steels, weld quality must be of the highest degree. Root fusion must be complete, and there should be no undercut or any type of stress risers. The weld metal should be free of porosity and any weld cracking is absolutely unacceptable. All precautions must be taken in order to produce the highest weld quality.

Solid-state welding

Like the first welding process, forge welding, some modern welding methods do not involve the melting of the materials being joined. One of the most popular, ultrasonic welding, is used to connect thin sheets or wires made of metal or thermoplastic by vibrating them at high frequency and under high pressure. The equipment and methods involved are similar to that of resistance welding, but instead of electric current, vibration provides energy input. Welding metals with this process does not involve melting the materials; instead, the weld is formed by introducing mechanical vibrations horizontally under pressure. When welding plastics, the materials should have similar melting temperatures, and the vibrations are introduced vertically. Ultrasonic welding is commonly used for making electrical connections out of aluminum or copper, and it is also a very common polymer welding process.

Another common process, explosion welding, involves the joining of materials by pushing them together under extremely high pressure. The energy from the impact plasticizes the materials, forming a weld, even though only a limited amount of heat is generated. The process is commonly used for welding dissimilar materials, such as the welding of aluminum with steel in ship hulls or compound plates. Other solid-state welding processes include co-extrusion welding, cold welding, diffusion welding, friction welding (including friction stir welding), high frequency welding, hot pressure welding, induction welding, and roll welding.

Energy beam welding

Energy beam welding methods, namely laser beam welding and electron beam welding, are relatively new processes that have become quite popular in high production applications. The two processes are quite similar, differing most notably in their source of power. Laser beam welding employs a highly focused laser beam, while electron beam welding is done in a vacuum and uses an electron beam. Both have a very high energy density, making deep weld penetration possible and minimizing the size of the weld area. Both processes are extremely fast, and are easily automated, making them highly productive. The primary disadvantages are their very high equipment costs (though these are decreasing) and a susceptibility to thermal cracking. Developments in this area include laser-hybrid welding, which uses principles from both laser beam welding and arc welding for even better weld properties.

Resistance welding

Resistance welding involves the generation of heat by passing current through the resistance caused by the contact between two or more metal surfaces. Small pools of molten metal are formed at the weld area as high current (1000–100,000 ) is passed through the metal. In general, resistance welding methods are efficient and cause little pollution, but their applications are somewhat limited and the equipment cost can be high.

Spot Welding is a popular resistance welding method used to join overlapping metal sheets of up to 3 mm thick. Two electrodes are simultaneously used to clamp the metal sheets together and to pass current through the sheets. The advantages of the method include efficient energy use, limited workpiece deformation, high production rates, easy automation, and no required filler materials. Weld strength is significantly lower than with other welding methods, making the process suitable for only certain applications. It is used extensively in the automotive industry—ordinary cars can have several thousand spot welds made by industrial robots. A specialized process, called shot welding, can be used to spot weld stainless steel. Like spot welding, seam welding relies on two electrodes to apply pressure and current to join metal sheets. However, instead of pointed electrodes, wheel-shaped electrodes roll along and often feed the workpiece, making it possible to make long continuous welds. In the past, this process was used in the manufacture of beverage cans, but now its uses are more limited. Other resistance welding methods include flash welding, projection welding, and upset welding.

Gas welding

The most common gas welding process is oxyfuel welding, also known as oxyacetylene welding. It is one of the oldest and most versatile welding processes, but in recent years it has become less popular in industrial applications. It is still widely used for welding pipes and tubes, as well as repair work. The equipment is relatively inexpensive and simple, generally employing the combustion of acetylene in oxygen to produce a welding flame temperature of about 3100 °C. The flame, since it is less concentrated than an electric arc, causes slower weld cooling, which can lead to greater residual stresses and weld distortion, though it eases the welding of high alloy steels. A similar process, generally called oxyfuel cutting, is used to cut metals.^[22] Other gas welding methods, such as air acetylene welding, oxygen hydrogen welding, and pressure gas welding are quite similar, generally differing only in the type of gases used. A water torch is sometimes used for precision welding of items such as jewelry. Gas welding is also used in plastic welding, though the heated substance is air, and the temperatures are much lower.

Arc welding

(DC) or These processes use a welding power supply to create and maintain an electric arc between an electrode and the base material to melt metals at the welding point. They can use either directalternating (AC) current, and consumable or non-consumable electrodes. The welding region is sometimes protected by some type of inert or semi-inert gas, known as a shielding gas, and filler material is sometimes used as well.

Power supplies

To supply the electrical energy necessary for arc welding processes, a number of different power supplies can be used. The most common classification is constant current power supplies and constant voltage power supplies. In arc welding, the length of the arc is directly related to the voltage, and the amount of heat input is related to the current. Constant current power supplies are most often used for manual welding processes such as gas tungsten arc welding and shielded metal arc welding, because they maintain a relatively constant current even as the voltage varies. This is important because in manual welding, it can be difficult to hold the electrode perfectly steady, and as a result, the arc length and thus voltage tend to fluctuate. Constant voltage power supplies hold the voltage constant and vary the current, and as a result, are most often used for automated welding processes such as gas metal arc welding, flux cored arc welding, and submerged arc welding. In these processes, arc length is kept constant, since any fluctuation in the distance between the wire and the base material is quickly rectified by a large change in current. For example, if the wire and the base material get too close, the current will rapidly increase, which in turn causes the heat to increase and the tip of the wire to melt, returning it to its original separation distance.

The type of current used in arc welding also plays an important role in welding. Consumable electrode processes such as shielded metal arc welding and gas metal arc welding generally use direct current, but the electrode can be charged either positively or negatively. In welding, the positively charged anode will have a greater heat concentration, and as a result, changing the polarity of the electrode has an impact on weld properties. If the electrode is positively charged, the base metal will be hotter, increasing weld penetration and welding speed. Alternatively, a negatively charged electrode results in more shallow welds.Nonconsumable electrode processes, such as gas tungsten arc welding, can use either type of direct current, as well as alternating current. However, with direct current, because the electrode only creates the arc and does not provide filler material, a positively charged electrode causes shallow welds, while a negatively charged electrode makes deeper welds.Alternating current rapidly moves between these two, resulting in medium-penetration welds. One disadvantage of AC, the fact that the arc must be re-ignited after every zero crossing, has been addressed with the invention of special power units that produce a square wave pattern instead of the normal sine wave, making rapid zero crossings possible and minimizing the effects of the problem.

Processes

One of the most common types of arc welding is shielded metal arc welding (SMAW), which is also known as manual metal arc welding (MMA) or stick welding. Electric current is used to strike an arc between the base material and consumable electrode rod, which is made of steel and is covered with a flux that protects the weld area from oxidation and contamination by producing CO₂ gas during the welding process. The electrode core itself acts as filler material, making a separate filler unnecessary.

The process is versatile and can be performed with relatively inexpensive equipment, making it well suited to shop jobs and field work. An operator can become reasonably proficient with a modest amount of training and can achieve mastery with experience. Weld times are rather slow, since the consumable electrodes must be frequently replaced and because slag, the residue from the flux, must be chipped away after welding. Furthermore, the process is generally limited to welding ferrous materials, though speciality electrodes have made possible the welding of cast iron, nickel, aluminium, copper, and other metals. Inexperienced operators may find it difficult to make good out-of-position welds with this process.

Gas metal arc welding (GMAW), also known as metal inert gas or MIG welding, is a semi-automatic or automatic process that uses a continuous wire feed as an electrode and an inert or semi-inert gas mixture to protect the weld from contamination. As with SMAW, reasonable operator proficiency can be achieved with modest training. Since the electrode is continuous, welding speeds are greater for GMAW than for SMAW. Also, the smaller arc size compared to the shielded metal arc welding process makes it easier to make out-of-position welds (e.g., overhead joints, as would be welded underneath a structure).

The equipment required to perform the GMAW process is more complex and expensive than that required for SMAW, and requires a more complex setup procedure. Therefore, GMAW is less portable and versatile, and due to the use of a separate shielding gas, is not particularly suitable for outdoor work. However, owing to the higher average rate at which welds can be completed, GMAW is well suited to production welding. The process can be applied to a wide variety of metals, both ferrous and non-ferrous.

A related process, flux-cored arc welding (FCAW), uses similar equipment but uses wire consisting of a steel electrode surrounding a powder fill material. This cored wire is more expensive than the standard solid wire and can generate fumes and/or slag, but it permits even higher welding speed and greater metal penetration.

Gas tungsten arc welding (GTAW), or tungsten inert gas (TIG) welding (also sometimes erroneously referred to as heliarc welding), is a manual welding process that uses a nonconsumable tungsten electrode, an inert or semi-inert gas mixture, and a separate filler material. Especially useful for welding thin materials, this method is characterized by a stable arc and high quality welds, but it requires significant operator skill and can only be accomplished at relatively low speeds.

GTAW can be used on nearly all weldable metals, though it is most often applied to stainless steel and light metals. It is often used when quality welds are extremely important, such as in bicycle, aircraft and naval applications. A related process, plasma arc welding, also uses a tungsten electrode but uses plasma gas to make the arc. The arc is more concentrated than the GTAW arc, making transverse control more critical and thus generally restricting the technique to a mechanized process. Because of its stable current, the method can be used on a wider range of material thicknesses than can the GTAW process, and furthermore, it is much faster. It can be applied to all of the same materials as GTAW except magnesium, and automated welding of stainless steel is one important application of the process. A variation of the process is plasma cutting, an efficient steel cutting process.

Submerged arc welding (SAW) is a high-productivity welding method in which the arc is struck beneath a covering layer of flux. This increases arc quality, since contaminants in the atmosphere are blocked by the flux. The slag that forms on the weld generally comes off by itself, and combined with the use of a continuous wire feed, the weld deposition rate is high. Working conditions are much improved over other arc welding processes, since the flux hides the arc and almost no smoke is produced. The process is commonly used in industry, especially for large products and in the manufacture of welded pressure vessels. Other arc welding processes include atomic hydrogen welding, carbon arc welding, electroslag welding, electrogas welding, and stud arc welding.

Welding

Welding is a fabrication process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material (the weld puddle) that cools to become a strong joint, with pressure sometimes used in conjunction with heat, or by itself, to produce the weld. This is in contrast with soldering and brazing, which involve melting a lower-melting-point material between the workpieces to form a bond between them, without melting the workpieces.

Many different energy sources can be used for welding, including a gas flame, an electric arc, a laser, an electron beam, friction, and ultrasound. While often an industrial process, welding can be done in many different environments, including open air, underwater and in space. Regardless of location, however, welding remains dangerous, and precautions must be taken to avoid burns, electric shock, poisonous fumes, and overexposure to ultraviolet light.

Until the end of the 19th century, the only welding process was forge welding, which blacksmiths had used for centuries to join metals by heating and pounding them. Arc welding and oxyfuel welding were among the first processes to develop late in the century, and resistance welding followed soon after. Welding technology advanced quickly during the early 20th century as World War I and World War II drove the demand for reliable and inexpensive joining methods. Following the wars, several modern welding techniques were developed, including manual methods like shielded metal arc welding, now one of the most popular welding methods, as well as semi-automatic and automatic processes such as gas metal arc welding, submerged arc welding, flux-cored arc welding and electroslag welding. Developments continued with the invention of laser beam welding and electron beam welding in the latter half of the century. Today, the science continues to advance. Robot welding is becoming more commonplace in industrial settings, and researchers continue to develop new welding methods and gain greater understanding of weld quality and properties.

Welding Certification and Welder Qualification Services from TCR

TCR Engineering Services has expanded its quality assurance and third party inspection services to include a comprehensive welder certification and welding procedure qualification program. As part of this enhanced service offering, TCR will undertake the following:

· Welder Qualification Testing for performance qualification and certification of welders (a welder / welding operator performance qualification - WQT) to ASME, ANSI, AWS, API code

· Preparation of Weld Procedure Qualification as per client or project requirements.

· Coupon Testing as per Weld Procedure Qualification which includes visual examination, mechanical testing, metallographic examination and non destructive testing.

· Documentation of the Procedure Qualification Record as per ASME, ANSI, AWS, API codes

· In depth weld inspection to include review of the applicable qualification e.g. weld procedure specification, welder performance qualification and validity for process materials and consumable items, equipment, set up and other factors, including certificates of calibration and/or conformity governing the work.

· Check safety of set up and operation having due regard for self, welder and other workers in vicinity, particular in respect of ultraviolet radiation from arc during welding.

The welding inspector deployed at a site from TCR will be responsible for monitoring and verifying that the following functions of the work conform in all aspects to the specific requirements of the relevant code, specification and/or standard:

• - Check correct weld procedure(s) employed.
• - Check weld procedure and welder qualifications.
• - Inspect weld profile preparation.
• - Inspect joint fit-up
• - Check filler metals and consumable materials
• - Check correct welding performance parameters observed
• - Perform visual examination upon completion of welding
• - Monitor pre and post weld heat treatment, where specified.

• - Monitor the physical examination including non-destructive test, hydrostatic test, mechanical test etc. If needed, the inspector may choose to send test coupons to the TCR Engineering Services' material testing laboratory.
• - Ensure that all necessary visual inspection is completed and verify that all other necessary non-destructive examinations are executed in the specified manner for the method and coverage by appropriately qualified personnel.

When, and where, required the inspector shall employ the following equipment to aid in the performance of duties:

• a) Inspection mirrors.
• b) Torch or other electrical lighting facilities (permitted by safety codes eg. 24V system etc.)
• c) Physical size measuring instruments such as welding gauge, rule, vernier etc.
• d) Electrical parameter measuring instruments such as ammeter, voltmeter etc.
• e) Temperature measuring instruments (thermometer)/aids (thermo chalk).

All TCR welding inspectors are generally certified in accordance with the requirements of at least one of the following schemes - Certification Scheme for Weld Inspection Personnel (CSWIP), American Welding Society (AWS), BGAS (previously British Gas ERS), and/or ASNT Level II VT. All inspectors have the ability to interpret various standards including ASME B&PV Code, Section IX, API Std. 1104 and ANSI / AWS D1.1.

The Welding Inspection department at TCR is headed by Mr. Nilesh Pathare who has over 11 years of experience in QA /QC inspection in oil and Gas industry, Petrochemical and refineries and is qualified as a CSWIP 3.1 Welding Inspector, AWS-CWI, BGAS- CSWIP Painting Inspector and ASNT LEVEL II UT, MT, PT, RT. He is experienced in pressure vessel fabrication (static equipment) inspection and Third Party Inspection of materials like plates, pipes, forgings, casting at a vendor's location. He also has hands on experience in NDT (UT, MT, PT) and Radiographic Film interpretation and Destructive testing of various materials.