KR20090112705A - Method for improving the performance of seam-welded joints using post-weld heat treatment - Google Patents

Abstract

The present invention provides a method for heat-treating seam-welded constructions of hardenable steel and ferrous alloys with reduced weld-zone hardness and improved weld-zone ductility and toughness. This method consists of heating the seam weld rapidly with a secondary heat source to a temperature greater than the martensitic start temperature but not greater than the lower critical temperature, followed by immediately allowing the seam weld to air-cool. The rapid tempering of this invention is particularly suited to the production of high strength hardenable alloy seam-welded pipe and tubing and other structures.

Description

METHOD FOR IMPROVING THE PERFORMANCE OF SEAM-WELDED JOINTS USING POST-WELD HEAT TREATMENT}

The present invention relates to a post weld heat treatment and method. In particular, the present invention relates to a method for improving the mechanical properties of seam welds with reduced weld site hardness and improved weld site ductility and toughness on hardenable ferrous alloys.

Welded ferroalloys used everywhere in all modern industries have become the de facto standard for structural component design. The current trend in many areas is shifting the focus of attention from low strength common mild steel to high strength and ultra high strength steel. These alloys are made to have greater tensile strength than low carbon steels because of the specific microstructures that are produced during the thermomechanical process. Some examples of high strength steels currently in use in the automotive industry include biphasic, maltensite, boronated and metamorphically induced plastic steels. Other high strength steels include air, oil and water hardened carbon steels and maltensite stainless steel. All of these are designed so that some volume percent of martensite form the microstructure of the material. The resulting distorted body core cubic system (BCC) or body core hexagonal system (BCT) maltensite crystal structure, which is formed in a hardened state, gives high strength to the metal. These materials are ideally positioned in structural materials and assemblies to meet the requirements for high strength and toughness.

Unfortunately, the tendency to form maltensite and relatively high hardness in these and other ultra high strength alloys presents disadvantages in welding. The heat cycle of heating and quenching occurring within the narrow heat affected zone during welding is the same as the rapid quenching cycle. High strength steel grade chemicals are completely transformed from ferrite to austenite (γ) at high temperatures and then continue to change to the hard martensite phase upon quenching. In seam welding applications, the natural weld cooling rate can be as high as 1000 ° C./sec. Fast enough to produce maltensite structures in the highest strength high carbon steels (see FIGS. 1 and 2). The resulting maltensite tissue is extremely brittle in the untempered state. Cracks in the weld zone can occur for several reasons, including:

■ Hydrogen-induced low temperature cracks due to hydrogen trapped within the distorted BCC maltensite crystal structure. Tensile stress applied to the weld increases the risk of cracking.

■ Heat induced stress due to heat input during welding, degree of joint restrain, and volume change during maltensite transformation.

Most forms of cracking are due to shrinkage deformation that occurs when the weld metal cools to ambient temperature. If shrinkage is limited, the strain will induce residual tensile stresses that are the cause of the cracks. There are two opposing forces, one is the stress caused by the shrinkage of the metal and the other is the peripheral hardening of the base metal. Large weld size, high heat input and deep penetration welding process increase shrinkage deformation. The stress induced by these deformations will increase when higher strength filler metals and base metals are included. In case of higher yield strength, higher residual stress will be present.

These problems arise when welding any steel, regardless of the preceding conditions with regard to annealing, hardening, or hardening and tempering. These problems can occur with all types of welding, including GTAW, GMAW SAW, PAW, laser beams, friction, resistance and electron beams. In all cases, the molten site and the hot HAZ will be "as-quenched" after welding. Any mechanical deformation after welding in the secondary manufacturing or maintenance process will cause the maltensite HAZ to crack.

In addition, many assemblies once welded and fabricated from these alloys will be placed in a heat treatment cycle for final homogenization. Examples include assemblies made from pre-hardened or certain heat-machined base metals, such as two-phase steel, whereby the specific microstructure of the alloy is destroyed by thermal cycling. In addition, positioning the entire welded assembly inside the furnace to remove post weld stresses reduces the physical potential as in the case of automotive structural beams welded to heavy vehicle body structures. Some assemblies may not be capable of post-welding heat treatment of the entire structure as in the case of a welded fuel tank assembly with thermoplastic internal parts. In some cases, a great benefit can be realized when the brittleness of the weld is reduced. Ductility and toughness of the finished weld will be significantly improved in the case of welded structures in use without any heat treatment inside the furnace.

Conventional methods of controlling welding and HAZ hardness include off-line secondary post-weld heat treatment (PWHT) by heating the entire portion, such as the annealing and tempering processes of the weld. As the preheating method, a method of reducing the presence ratio of maltensite may be used by slowing down the cooling rate. Potential heat in the workpiece prevents cracking by reducing the cooling rate of the welded seam. In the past, pre- and post-weld heat treatments have been performed in large batch heat treatment furnaces to maintain and process a group of components at suitable heat treatment temperatures. The disadvantage of using a batch heat treatment process is that it takes a long heat treatment time, in part due to the size of the large batch furnace and the size of the parts to be heat treated. In addition, a long waiting period is required while assembling the batch furnace while welding the individual parts. Standard post-weld heat treatments, such as stress relief or tempering, require slow furnace cooling rates and a fairly long holding time of several hours at predetermined temperatures. In a more complex manner it is possible to destroy certain microstructures of the base metal by full pre- or post-weld heat treatment. For example, parts made of biphasic or maltensitic steel lose overall mechanical properties when the entire part is heat treated differently from the optimal heating time and quenching ratio.

It is well known to the producers to cool the joints slowly in air when re-austizing the seam welds by oxyacetylene torch and processing hardened steels such as chromium-based alloys. The heat retained by the weld joint and the surrounding metal effectively slows down the cooling rate of the weld after reheating, reducing brittleness and leaving the joint flexible and steady. However, the process is variable and slow and depends on the skill of the manufacturer as a separate result. Another method, as described in US Pat. No. 3,046,167, entitled Heat Treatment Methods and Products, is provided for slow cooling after re-austenitic welding seams by a torch. A similar method is described in US Pat. No. 6,676,777, entitled Post Weld Heat Treatment of Carbon and Low Alloy Steels, which describes a method of reheating a weld to an austenite zone, maintaining it for several hours, and then further cooling in air. Doing. All these methods are to produce the desired microstructure by reheating and slow cooling the welding seam above the upper critical temperature.

Conventional controlled “local” post-weld stress relief methods have often been applied to poorly welded plumbing weld joints in the service industry and oil sectors. Heat is applied through heating "blankets" consisting of inductive or resistive heating coils wound around the circumference of the joint. The heat is applied very slowly, allowing it to stay at the peak temperature for a few minutes to several hours and allow it to cool very slowly in the isolation heating blanket.

In seam welded tube products, a common approach to solve the inherent welding difficulties of high strength steels is to change the chemical composition of the material. Typically, the development of low-carbon alloys of hardened alloys in air has resulted in seam welding that is not sufficiently martensitic and no cracking occurs during tube manufacture. An example thereof is described in US Pat. No. 7,157,672, entitled Method for Manufacturing Stainless Steel for Use in Piping Systems, which details the use of up to 0.08% C stainless steel in low carbon two phases in conventional tube manufacturing processes. Explaining. Similarly, a modified composition was used to fabricate a pipe in the article, The Development of Weldable Maltensite Stainless Steel Line Pipes by the HF-ERW Process, published by Stainless Steel World 1999 Conference Procedure, Ayukawa et al. In 1999. In changing the chemical composition there is a compromise between ease of welding and the hardenability and maximum mechanical properties of the material.

Another way to reduce the weld hardness is to modify the final metallization in such a way as to reduce the proportion of hard and brittle components such as maltensite by adding filler material. However, it is difficult to use filler metal in some seam welding processes (such as laser or resistance welding). In addition, expensive filler metals are selected to not cure upon cooling, thereby providing a weld of lower strength. This requires the use of much larger welds to meet the required joint strength.

Another method for improving seam welding properties includes mechanically deforming and machining the weld to reduce the cracking tendency of the weld seam by inducing residual compressive stress. This method is not only efficient, but only in the simplest seam weld geometry, not in all cases. This method is described in detail in US Pat. No. 4,072,035, titled Reinforcing Weld Seams.

The use of "seam annealer" to improve the mechanical properties of seam welding is well known in the field of seam welded tube products. Designed to operate on unhardened alloys such as low carbon steel and austenitic stainless steel, these devices apply a secondary heat source on the seam weld downstream of the welding source after being sufficiently cooled to ambient temperature. Two main features apply: first, the "seam annealer" reheats the weld above the A _C3 temperature, retains the material for several hours to re-austenitize the material and slowly cools equally to the "normalizing" heat treatment; "Sim anneal" is used for uncured alloys. Examples of “seaming annealing” processes are described in US Pat. No. 3,242,299, entitled Induction Machines for Induction Heating Devices, and US Pat. No. 4,975,128, titled Methods for Heat Treatment of Bead Welded Pipes for Use in Piping Systems. .

US Pat. No. 2,293,481, entitled Welding Apparatus, and US Pat. No. 2,262,705, named Electrical Welding, describe a method for manufacturing welds with reduced brittleness. Both methods use a fairly short tempering cycle for the hardened alloy and reheat the weld to improve mechanical properties. However, these methods are used for band saw blades and spot welding is different from the present invention. These processes are performed in situ using the same equipment used to create the weld. In fact, the spot welding equipment is reheated to temper the weld immediately after staying in place for proper quenching for the method of US Pat. No. 2,262,705. Most importantly, these processes are used for separate welding joints, namely spot, flash, or projection type welding.

Conventional processes such as batch preheating and PWHT on their own are undesirable in terms of cost efficiency, high quality and high volume manufacturability. Unfortunately, these methods are undesirable in terms of cost, time as well as energy efficiency for the high manufacturing levels associated with current manufacturing methods. An ideal solution is to enable or enable autogenous welding of harmonized strength filler metal with a chemical composition similar to that of the base metal to be welded, which can be hardened into a high strength joint, combined with quench and inexpensive air cooling cycles. will be.

The inventors disclose US Provisional Application No. 7,232,053, issued June 19, 2007, US Provisional Application No. 60 / 879,861, filed January 10, 2007, US Application No. 11 / 542,970, filed October 4, 2006. US Application No. 11 / 526,258, filed September 22, 2006, US Application No. 11 / 519,331, filed September 11, 2006, US Application No. 10 / filed December 30, 2004. 519,910, International Application No. PCT / US02 / 20888, filed Jul. 1, 2002, US Provisional Application No. 60 / 301,970, filed June 29, 2001, show various methods for increasing welding and HAZ ductility. Explaining. Each of these references is incorporated herein by reference in its entirety. Unfortunately, even these methods have disadvantages.

Therefore, it would be desirable from a manufacturing point of view to preferentially provide heat treatment during manufacture to improve the mechanical properties of the seam welded joint. Preferably, a simple PWHT method can be used to effectively increase welding and HAZ ductility.

In summary, according to the present invention there is provided an improved method for forming an all-inclusive steel structure, without being limited to welded tube structures. In its broadest term, the present invention relates to an improved post-weld heat treatment (PWHT) for hardened alloy iron.

The method of forming the steel structure of the present invention includes processing a conventional weld formed when two surfaces of hardened alloy iron are welded together. The initial weld is formed by applying a heat source, preferably in the form of a conventional welding device, to join adjacent surfaces at a temperature high enough to melt the alloy iron to form a weld. The weld is then cooled to below the martensite starting point M _S of hardened alloy iron. The weld can be cooled to ambient temperature. Alternatively, the weld can be cooled to an intermediate temperature between the martensite starting point M _S of the ferroalloy and the ambient temperature.

After cooling the seam weld to a temperature below the martensite starting point M _S , the seam weld is tempered in a post weld heat treatment process. The seam weld is quenched above the martensite starting point (M _S ) temperature of the hardened alloy iron of the weld at a rate of 10 ° C. or more per second. However, the weld is not heated above the lower critical temperature A _C1 of the hardened alloy iron of the weld. What is important is that the welds are quenched at a rate of at least 10 ° C. per second, preferably 200 ° C. per second. Heating to the weld seam is preferably a local heat source but can be performed using a variety of heat sources. Local heat sources include, but are not limited to, propane or oxyacetylene torches, resistors, electric arcs, lasers, conduction, radiation, convection, or high frequency induction methods. The local heat source described in the present invention provides heat to the weld and its adjacent areas but does not heat the entire part.

Once the seam weld is tempered by heating the seam weld above the martensite starting point temperature M _S for hardened ferroalloy but above the lower critical temperature A _C1 , the seam weld is free of soak time at the holding temperature. Air cooled immediately. Air quenching is performed at least 15 ° C. per minute to preferably less than 200 ° C. per second as provided by water cooling.

A wide range of hardened alloyed irons including iron alloys and alloys curable in air can be used in embodiments of the present invention. The method for post weld heat treatment and the method of forming the steel structure of the present invention can be particularly applied to hardened maltensite stainless steel in the form of 410, 420 and 440. Different alloys have different lower critical temperatures, maltensite start point (M _S ) temperature, and maltensite end point (M _F ) temperature, and the welding properties can be changed according to the welding design, so the tempering rate, tempering final temperature and cooling rate Will also change.

The method of forming the steel structure of the present invention is also particularly applicable to forming seam welded pipe and piping structures as well as to forming circumferential welds, such as around gas or liquid tanks.

It is therefore an object of the present invention to provide an improved method for forming a welded steel structure of hardened ferroalloy.

Another object of the present invention is to provide an improved method of forming a steel structure in which heat treatment is performed during initial fabrication to improve the mechanical properties for a seam welded joint.

Another object of the present invention is to provide a weld heat treatment after increasing the welding and HAZ ductility without increasing the processing time.

It is another object of the present invention to provide a method of forming a steel structure in an inexpensive and fairly simple manner.

These and further objects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description taken in conjunction with the drawings.

1 is a chart illustrating four separate microstructure zones observed in the heat affected zone (HAZ) of an air hardenable steel after welding,

FIG. 2 is a graph of fine hardness across conventional welds of air hardenable maltensite stainless steel that has not been heat treated before or after;

3 is a graph of the post weld heat treatment temperature profile for conventional tempering compared to the temperature profile of the present invention,

4 is a flowchart illustrating a post-weld heat treatment process of the present invention,

5 is a typical configuration of pre-hardened shells prior to flange seam welding.

An enlarged perspective view of a clamshell tank assembly,

6 is a perspective view of a conventional clamshell tank assembly consisting of a pre-cured shell after flange seam welding,

7 is a perspective view of a conventional clamshell tank assembly after flange seam welding showing local weld seam heat treatment with a formed resistance heating coil;

8 is a perspective view of a conventional clamshell tank assembly after flange seam welding showing local weld seam heat treatment by the formed resistance heating coil in use,

9 is a cross-sectional view of a typical clamshell tank assembly after flange seam welding showing local weld seal heat treatment by a heating coil (resistive or inductive) located within the clamping fixture;

10 is a cross-sectional view of a conventional clamshell tank assembly after flange seam welding showing local weld seam heat treatment by passing an electric current through a shell for local weld seam heat treatment;

FIG. 11 is a bar graph comparing the maximum strain at break for DIN EN 895 longitudinal tensile specimens of a 410 type seam welded sample treated according to the invention with an average result of 62 trials.

Hereinafter, embodiments of the present invention will be described in various forms as shown in the drawings, but it should be understood that the preferred embodiments of the present invention are considered as examples of the present invention, and the present invention is not limited to the specific embodiments described. .

The present invention includes methods for treating seam weld structures and seam welds made from high strength steel and other hardenable alloys. The present invention is particularly applicable to alloys that transform into martensitic welds and HAZ microstructures in seam welding, and the following description will therefore focus on such steels. The process of the present invention improves the ductility and toughness of the weld zone and reduces brittleness and reduces the susceptibility to hydrogen induced low temperature cracks in the weld zone. The present invention improves mechanical deformation and deformation of weld zones, melt zones, and HAZ, and eliminates the need for additional post-weld heat treatment for the entire structure, such as annealing processes, sub-critical annealing or stress reduction processes. Remove The local heat treatment of the present invention eliminates the risk of altering the material properties and microstructure of the base metal, thereby allowing the invention to be suitable for alloys not subjected to pre-weld heat treatment as well as to heat treatment of the entire part. .

As summarized in the SAEJ412 specification, the general properties of steel and the heat treatment, curability or reactivity to heat treatment are one of the most important criteria in selecting steel. Curability is a property of steel that determines the depth and distribution of hardness induced by quenching over the transformation range. The term cure means that the hardness of the material is increased by a suitable heat source that generally includes heating to a suitable austenitization temperature followed by cooling to any minimum rate depending on the alloy content. When quenching is complete, the final structure becomes maltensite and its hardness depends on the carbon content of the steel.

As defined hereinafter, "hardenable alloy" refers to the degree of hardening of steel and ferroalloy in response to heat treatment. The "hardenable alloy" also has a sufficient carbon content with respect to other alloying elements, to form maltensite microstructures during melting and to form HAZs after conventional seam welding. As defined hereinafter, a "curable alloy" refers to a specific chemistry of the alloy, including A _C3 -upper threshold temperature, A _C1 -lower threshold temperature, M _S -maltensite start temperature, and M _F -maltensite end temperature. It has a well defined transformation temperature depending on the composition. A "hardenable alloy" includes steels and alloys that are considered hardenable in air because the reasonable quenching cooling rate associated with seam welding is greater than that in air. As defined herein, the term "hardenable alloy" includes steel and ferroalloy, which are determined to be a "low carbon carbonizing grade" that only reacts to heat treatment by melting elements to the material surface through a case hardening process. It doesn't work.

Representative curable alloys to which the present invention can be applied include,

SAE 1030,1034,1035,1037,1038,1039,1040,1042,1043,1044,1045,

1046,1049,1050,1053,1536 (1036), 1541 (1041), 1547 (1047), 1547 (1047),

1548 (1048), 1551 (1051), 1552 (1052)

SEA 1055,1059,1060,1064,1065,1069,1070,1074,1075,1078,1080,

1084,1085,1086,1090,1095,1561 (1061), 1566 (1066), 1572 (1072)

SEA 1330,1335,1340

SEA 4037,4047,4130,4135,4137,4140,4142,4145,4150,4161,4340

SEA 5046,50B40,50B44,50B46,50B50,5060,50B60,

SAE 5130,5132,5135,5140,5145,5147,5150,5155,51B60

SAE 6150

SAE 8630,8637,8640,8642,8645,8650,8655,8660,8740

SAE 81B45,86B45,94B30

SAE 9254,9255,9260

SAE 50 100, 51 100, 52 100

SAE 51410,51414,51420,51431,51440A, 51440B, 51440C, 51501

22MnB5

30MnB5

DP600

DP800

DP1000

A preferred invention of the present invention includes performing seam welding for joining two surfaces of hardenable maltensite steel. The seam weld may be cooled to below the martensite starting temperature M _S of the weld. The temperature of the weld may or may not be cooled to below the martensite end temperature or even to ambient temperature. As shown in Figs. 3 and 4, the seam welds completed thereafter are brought to a lower intermediate temperature higher than the A _C1 (bottom critical temperature) (eutectoid temperature), or maltensite starting (M _S ) temperature of the weld metal. It is rapidly heated, and the weld is air cooled. In the first embodiment described by the method “A” in FIG. 3 and shown by the dashed line on the temperature versus time chart, the seam weld is A _C1. Heated to temperature A _C1 Does not exceed the temperature (lower threshold temperature). This embodiment provides the maximum softening of the seam welds because all maltensite undergoes maximum hot tempering. In the second embodiment, which is described as method “B” in FIG. 3 and shown in dashed lines on a temperature vs. time chart, brittleness is achieved by heating to an intermediate temperature without excessive softening and without excessively reducing the tensile strength of the weld. By reducing it serves to improve the toughness of the seam weld.

Rapid heating of the seam weld is carried out at a rate greater than 10 ° C / sec. Preferably, the rapid heating of the seam weld is carried out much more rapidly at about 200 ° C./sec. Immediately after rapid heating (without soak time at holding temperature) air quenching is performed. The immediate transition of air cooling after rapid heating means that it can be inferred to cover quite broadly the transition cycle of seconds or even moisture because it can occur incidentally to the manufacturing process. However, the instantaneous transition cycle from rapid heating to air cooling does not mean that the isothermal soak time allows for a significant change in the crystal microstructure of ferroalloys such as carbide segregation and recrystallization. Preferably, the quench ratio consistent with air cooling is greater than 15 ° C./minute but less than 200 ° C./second.

The seam welds may be heated entirely by a heating source, or may be heated separately (as is the case for continuous seam welds on a mill) in the present invention. Heat is applied to the seam weld using any of a variety of local heat sources, including but not limited to propane or oxyacetylene torches, resistance, electric arc, laser, conduction, radiation, convection, or high frequency induction methods. The term "local" is used to describe a heat source that provides heat to a localized area of the part rather than heating the entire part as provided by a furnace or oven. In the case of continuous processes such as in the production of seam welded pipes and tubing, selectively heating the local seam welded area would be the most effective embodiment for larger pipes. Alternatively, as in helical induction coils or other means, the annular heating of the entire circumference of the pipe may also be included within the spirit of the invention. Such annular heating is believed to be much more suitable for small diameter pipes and piping. 5 and 6, which are preferred embodiments, the entire seam weld can be simultaneously heated in various configurations, including peripheral welding, such as in fuel or liquid tanks of curable alloys.

In addition to altering the hardness of the weld zone (ie, reducing and / or tempering the amount of maltensite present in the weld zone microstructure), several other HAZ cracking factors may be included that mitigate the effects of the process as follows. .

Allowing additional time for hydrogen diffusion and release during heating in the process. The hydrogen retained in this way when tensile stress is applied or remains causes hydrogen-induced low temperature cracking in the maltensite microstructure.

• elimination of shrinkage deformation and stress in the weld due to a reduced thermal gradient along the length of the weld.

● Increased melt area and HAZ ductility and toughness.

Tempering of the formed welds and any maltensite in the HAZ.

As shown in Figures 7 and 8, the formed resistance heating coil can be used to locally heat treat seam welds around the entire tank. The heating coil is applied from the top side, bottom side, or both sides and is positioned away from the seam welding surface or held in place by direct contact until the peak temperature is reached. The coil is then removed and the seam weld is air cooled to room temperature. However, no protective atmosphere is required, and if necessary the process can be carried out in a non-oxidizing atmosphere.

Alternatively, heat may be applied to the seam weld using an induction coil or flame or other method formed with reference to FIGS. 9 and 10. The heating coil may be encapsulated in a press die-structure (FIG. 9) that prevents seam welds from bending during the processing process. In another embodiment of the heating process shown in FIG. 10, a current passes through one part to another to resistively heat the seam weld to an appropriate temperature.

Typical automotive structural materials that require seam welding of prehardened curable alloys include chassis components, A, B and C pillars, roof rails, roof bows, impact beams and bumpers. The size and range of the final body assembly hinders post weld stress reduction of any sufficient structural material. The post weld heat treatment process of the present invention is ideally suited for improving the performance of weld joints in these and similar types of applications.

Indeed, initial experimental results on air hardenable maltensitic stainless steel have shown significant improvements in ductility and toughness of seam welds treated according to the first and second embodiments described above when compared to as-welded specimens. Able to know. For example, experiments according to the invention were performed on seam weld strips using 410 stainless steel (UNS41000, SAE51410) types of 0.5 mm, 1.0 mm, and 2.0 mm thickness. Linear seam welding test fixtures were designed with test strips of 60 inches in length in butt GTAW welding, and experimental coupons therefrom were taken at the center constant welding site. A linear induction coil on one side was installed downstream of the GTAW torque body in accordance with the present invention. Direct temperature measurements were performed using an infrared pyrometer with a non-contact 3.9 μm wavelength.

The seam welds were allowed to cool to 180 stainless steel types at or below Maltensite Start Temperature (M _S ), which is approximately 330 ° C. and 230 ° C., as well as to approximately 180 ° C., which is below Maltensite End Temperature (M _f ). The completed seam weld was then quenched to approximately 650 ° C., just below the lower critical temperature A _C1 , which was approximately 675 ° C., and was immediately air cooled. As shown in FIG. 11, drawn at the maximum elongation in the seam welding (longitudinal tensile) direction, the same welded specimen showed significant improvement in welding and HAZ ductility by tempering according to the process of the present invention. The greatest benefit can be seen in thicker test strips, because thicker gauges, which alleviate the heat treatment process of the present invention, impose a greater degree of seam weld joint suppression, thereby creating a greater residual stress. Because it imposes. When different alloys are treated, the application of (A _C1 ) (lower critical temperature (vacancy temperature)) or heat to the lower intermediate temperature to the weld provides different mechanical properties for the welded area and thus the material to be used and the desired Depending on the mechanical properties of the material can be selected.

The present invention is ideally suited for all welding processes such as laser welding, resistance seam welding, and arc welding. The present invention is also ideal for treating seam welds using hardenable weld filler alloys in addition to native seam welds to reduce the brittleness of the melt zone and HAZ. The choice of A _C1 -bottom critical temperature (vacancy temperature) and M _S -maltensite starting temperature at which the ferrite and carbide are stable at the upper threshold temperature, below, depends on the chemical composition of the welding and base alloy. The natural cooling rate depends on the material thickness, joint shape, alloy form and ambient conditions.

While certain forms of the invention have been shown and described, it will be apparent that various changes may be made without departing from the spirit and scope of the invention. Therefore, it is to be understood that the invention is not intended to be further limited except as by the following claims. Since the present invention has been described to enable those skilled in the art to understand the present invention, it is to be understood that the present invention can be modified and practiced to identify the present invention and embodiments.

Claims (13)

As a method of forming a steel structure, Providing a first surface of hardened alloy iron, Providing a second surface of hardened alloy iron, Positioning the first surface adjacent to the second surface; The first and second surfaces by applying a first heat source to the first and second surfaces at a high temperature sufficient for the first and second surfaces to be above the melting point of the first and second surfaces to form a seam weld; 2 seam welding the surface, Cooling the seam weld to below the maltensite starting temperature for the hardened alloy iron, Tempering the seam weld, subsequent to cooling the seam weld to below the martensite start temperature for the hardened alloy iron, above the martensite start temperature but at a rate of 10 ° C. or more per second Tempering the seam weld, comprising heating the seam weld below a lower critical temperature for alloy iron, and Air cooling the seam weld at a rate of 15 ° C. per minute or greater after tempering the seam weld; Method of forming steel structures. The method of claim 1, Tempering the seam weld is performed at a rate ranging from 10 ° C. per second to 200 ° C. per second and air cooling the seam weld is performed at a rate ranging from 15 ° C. per minute to 200 ° C. per second, Method of forming steel structures. The method of claim 1, Tempering the seam weld includes using a local heat source to heat the seam weld; Method of forming steel structures. The method of claim 3, wherein Tempering the seam weld is performed at a rate ranging from 10 ° C. per second to 200 ° C. per second and air cooling the seam weld is performed at a rate ranging from 15 ° C. per minute to 200 ° C. per second, Method of forming steel structures. The method of claim 1, The hardened alloy iron is a maltensitic stainless steel curable in air with a carbon content of 0.08% by weight or more, Method of forming steel structures. The method of claim 1, Each of the hardened alloy iron is a maltensite stainless steel of the type 410, 420 or 440, Method of forming steel structures. The method of claim 1, Roller shaping the steel structure into a predetermined shape; The roller forming step occurs after welding the first and second surfaces together and tempering the weld, Method of forming steel structures. The method of claim 7, wherein Said steel structure is a tube and said first surface forms a first edge of a roll forming strip of alloy iron and said second surface forms a second edge of a roll forming strip of alloy iron; Method of forming steel structures. As a method of forming a steel structure, Providing a first surface of hardened alloy iron, Providing a second surface of hardened alloy iron, Positioning the first surface adjacent to the second surface; The first and second surfaces by applying a first heat source to the first and second surfaces at a high temperature sufficient for the first and second surfaces to be above the melting point of the first and second surfaces to form a seam weld; 2 seam welding the surface, Cooling the seam weld to below the maltensite starting temperature for the hardened alloy iron, Tempering the seam weld portion subsequent to cooling the seam weld portion to below the martensite starting temperature for the hardened alloy iron, the maltensite by local heat source at a rate ranging from 10 ° C. per second to 200 ° C. per second. Tempering the seam weld, comprising heating the seam weld above a start temperature but below a lower critical temperature for the hardened alloy iron, and Immediately air cooling the seam weld at a rate of 15 ° C. per minute to 200 ° C. per second after tempering the seam weld; Method of forming steel structures. The method of claim 9, The hardened alloy iron is a maltensitic stainless steel curable in air with a carbon content of 0.08% by weight or more, Method of forming steel structures. The method of claim 9, Each of the hardened alloy iron is a maltensite stainless steel of the type 410, 420 or 440, Method of forming steel structures. The method of claim 9, Roller shaping the steel structure into a predetermined shape; The roller forming step occurs after welding the first and second surfaces together and tempering the weld, Method of forming steel structures. The method of claim 12, Said steel structure is a tube and said first surface forms a first edge of a roll forming strip of alloy iron and said second surface forms a second edge of a roll forming strip of alloy iron; Method of forming steel structures.

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