HK1140521A - Method for improving the performance of seam-welded joints using post-weld heat treatment - Google Patents

Description

method for improving seam weld head performance using post-weld heat treatment

Background

The invention relates to a postweld heat treatment and a method. More particularly, the present invention is directed to a method of improving the mechanical properties of a weld on a hardenable ferrous alloy with reduced weld hardness and improved weld ductility and toughness.

Welded ferrous alloys, which are commonly used in all modern industries, have become the de facto standard in structural component design. The current trend in many fields is to shift focus from low strength plain low carbon steels to high strength steels and ultra high strength steels. These alloys are configured to have higher tensile strength than low carbon steels due to the specific microstructure created during thermomechanical processing. Some examples of high strength steels currently used in the automotive industry include dual phase steels, martensitic steels, boron treated steels, and transformation induced plasticity steels. Other high strength alloys include air, oil and water hardenable carbon steels and martensitic stainless steels. All these steels are designed so that several volume percent martensite is formed in the microstructure of the material. The resulting distorted Body Centered Cubic (BCC) or Body Centered Tetragonal (BCT) martensite crystal structure formed under quench hardening conditions imparts high strength to the metal. These materials are ideally suited for structural components and assemblies, meeting high strength and toughness requirements.

Unfortunately, the tendency of these and other ultra-high strength alloys to form martensite and the relatively high hardenability create difficulties in welding. The thermal cycle of heating and rapid cooling that occurs in the limited Heat Affected Zone (HAZ) during welding is equivalent to a rapid quench cycle. The chemical composition of high strength steel grades results in a complete transformation from ferrite to austenite (γ) at high temperatures, followed by rapid cooling and subsequent transformation to the hard martensite phase. In seam welding applications, the natural weld cooling rate can be as high as 1000 ℃/s, which is fast enough to produce a martensitic structure in most high strength, high carbon alloys (see fig. 1 and 2). The resulting martensitic structure is extremely brittle in the untempered state. Weld cracking can occur for several reasons, including:

■ hydrogen cools the crack, due to the hydrogen trapped in the distorted BCC martensite crystal structure. The tensile stress applied to the weld increases the risk of cracking.

■ thermally induced stresses due to heat input during welding, the degree of joint restraint, and volume changes at the martensitic transformation.

Most forms of cracking result from the shrinkage strains that occur when the weld metal cools to ambient temperature. If the shrinkage is limited, the strain will induce residual tensile stresses that lead to cracking. There are two opposing forces: stress caused by metal shrinkage, and stress caused by the surrounding rigidity of the base material. Large weld sizes, high heat input, and deep penetration welding operations increase shrinkage strains. These strain induced stresses will increase when higher strength filler metals and base materials are involved. For higher yield strengths, higher residual stresses may be present.

These problems occur when welding certain steels, regardless of their previous state, whether annealed, hardened or both hardened and tempered. They can occur with all types of welding including GTAW, GMAW, SAW, PAW, laser beam welding, friction welding, resistance welding and electron beam welding. In all cases, the fusion zone and high temperature HAZ will be in a "quenched" state after welding. Any mechanical strain after welding (either during secondary manufacturing or in service) can cause martensitic HAZ cracking.

In addition, many components, once welded and fabricated from these alloys, cannot be subjected to a final homogenizing solution heat treatment cycle. Examples include components made from pre-hardened or special thermomechanically treated base metals such as dual phase steel, whereby thermal cycling will destroy the unique microstructure of the alloy. Furthermore, as in the case of automotive structural beams welded to heavy vehicle bodies, placing the entire welded assembly into an oven for post-weld stress relief may not be practically (physically) feasible. Some components cannot withstand the post weld heat treatment of the entire structure, as is the case for welded automotive fuel tank components having thermoplastic interior components. In any case, significant benefits can be obtained if the brittleness of the weld can be reduced. In case the welded components are put into use without any additional heat treatment, the ductility and toughness of the final weld will be greatly improved.

Typical methods of controlling the weld and HAZ hardness include off-line secondary post-weld heat treatment (PWHT), such as intermediate annealing and tempering the weld by heating the entire part. A pre-heating method may be used to slow the cooling rate, thereby reducing the percentage of martensite phase present. Latent heat in the workpiece reduces the cooling rate of the weld, and cracking is thus suppressed. In the past, pre-and post-weld heat treatments have been performed in large batch heat treatment furnaces to ramp and maintain a group of components at a suitable heat treatment temperature. Drawbacks of using batch heat treatment processes include long treatment times, due in part to the size of the large batch furnace (mass) and the size of the parts being heat treated. In addition, long queuing times occur when batches are assembled as individual components are welded. Standard post-weld heat treatments such as stress relief or tempering involve relatively long holding times of about several hours at a specified temperature, with slow furnace cooling. To complicate matters, integral pre-or post-weld heat treatment can destroy the desired microstructure of the base metal. For example, parts made from dual phase or martensitic steels may suffer from a total loss of mechanical properties if the entire part is heat treated, except at the optimum heating time and quenching rate.

It is well known in manufacturing plants to reheat welds with oxyacetylene torches when working hardenable steels such as chrome molybdenum steel (chromoly) to re-austenitize the welds and allow the joints to cool slowly in static air. The retained heat of the welded joint and surrounding metal effectively slows the cooling rate of the weld after reheating, thereby reducing brittleness and leaving the joint in a milder normalized condition. However, this method is variable, slow, stage-by-stage and the results are highly dependent on the skill level of the manufacturer. Alternative methods, such as those described in U.S. Pat. No.3,046,167, entitled Heat-Treating Method and Product, provide for re-austenitizing the weld with a torch and then slowly cooling. A similar Process is described in U.S. Pat. No.6,676,777, entitled Postweld Heat Treatment Process of Carbon Steel and Low alloy Steel, which provides for reheating a weld to the austenitic region and holding for some time, followed by "slower than air" cooling. All of these methods rely on heating the weld above the upper critical temperature and slow cooling to produce the desired microstructure.

Conventional controlled "localized" post weld stress relief is typically applied to harsh-use industrial and oilfield pipeline weld joints. Heat is applied by a heating "blanket" consisting of an inductive or resistive heating coil surrounding the joint. Heat is applied very slowly, allowed to dwell for a few minutes to a few hours at peak temperature, and then allowed to cool very slowly in an insulating heating blanket.

In seam welded pipe production, a conventional approach to addressing the welding difficulties inherent in high strength alloys is to modify the chemical composition of the material. Typically, low carbon forms of air hardenable alloys have been developed so that the weld does not become fully martensitic and will not crack during pipe production. An example of such a process is U.S. patent 7,157,672 entitled Method of Manufacturing Stainless Steel Pipe for use In Pipe Systems which details the use of a low carbon dual phase, up to 0.08% C, Stainless Steel material In a conventional Pipe Manufacturing process. Similarly, the improved composition is used in the following articles to produce pipes: a Development of the flexible design stage line pipe by HF-ERWprocess, N.Ayukawa et al, Stainless stage World 1999 conference proceedings, 1999. In improving the chemical composition, there is a trade-off between ease of welding and hardenability and maximum mechanical properties of the material.

Another method of reducing weld hardness is to add filler material, thereby improving the final metallurgical properties such that the percentage of hard and brittle components, such as martensite, is reduced. However, some seam welding methods (e.g., laser welding or resistance welding) are difficult to use with filler metals. In addition, expensive filler metals are selected so as not to harden upon cooling, thereby providing a lower strength weldment. This makes it necessary to use even larger welds to meet the required joint strength.

Other methods of improving weld performance include mechanically deforming (strain) and working the weld to induce residual compressive stresses, thereby reducing the propensity of the weld to crack. This approach is not effective, or even feasible, for all weld geometries (except for the simplest). U.S. Pat. No.4,072,035, entitled Stronghening of a Welding team, details this process.

It is known in the production of seam-welded pipes to use "seam annealers" to improve the mechanical properties of the seam. These devices, designed to operate on non-quenched alloys such as mild steel and austenitic stainless steel, apply a secondary heat source to the weld bead downstream of the weld source (weld source) after it has cooled sufficiently to ambient temperature. Two main features apply: first, the "weld annealer" reheats the weld to above A_C3Re-austenitizing the material and holding for a certain time, then allowing slow cooling, equivalent to a "normalizing" heat treatment cycle; second, a "weld annealing apparatus" was used for the non-hardened alloy. Examples of "weld annealing" processes are described in U.S. Pat. No.3,242,299 entitled Inductor For Induction Heating Apparatus and U.S. Pat. No.4,975,128 entitled Method For Heat-Treating Straight welding For Use in Ping Systems.

U.S. Pat. No.2,293,481 entitled Welding Apparatus and U.S. Pat. No.2,262,705 entitled Electric Welding describe methods of producing welds with reduced brittleness. Both of these methods employ a relatively short tempering cycle on the hardenable alloy, reheating the weld to improve mechanical properties. However, these methods used on the band saw blade and spot welds are different from the present invention. These methods are performed in situ using the same equipment used to create the weld. In fact, for the method of U.S. Pat. No.2,262,705, the spot welding apparatus must stay in place for proper quenching, followed by immediate reheating to temper the weld. Most notably for discontinuous weld joints, i.e. spot, flash or bump welds.

Conventional methods such as batch preheating and PWHT are not suitable for low cost, high quality, high volume production. Unfortunately, these methods are not cost, time, or energy efficient for the high production levels associated with modern manufacturing methods. The ideal method would allow for self-fluxing welding (i.e., no filler metal used) or the use of a filler metal of compatible strength with the base metal being welded that is capable of hardening high strength joints with a similar chemical composition and combined rapid heating and inexpensive air cooling cycles.

The inventors describe various methods of improving weld and HAZ ductility in the following patents: U.S. patent No.7,232,053 issued on 6/19/2007; U.S. provisional patent application No.60/879,861 filed on 10.1.2007; U.S. application No.11/542,970 filed on 4.10.2006; U.S. application No.11/526,258 filed on 22/9/2006; U.S. application No.11/519,331 filed on 11/9/2006; U.S. application No.10/519,910 filed on 30.12.2004; international application No. pct/US02/20888 filed on 7/1/2002; U.S. provisional application No.60/301,970 filed on 29/6/2001. Each of these references is incorporated by reference herein in its entirety. Unfortunately, even these methods have drawbacks.

From a production point of view, it is therefore desirable to provide a heat treatment during production which may be preferred in order to improve the mechanical properties of the seam weld joint. Preferably, a simple PWHT method can be used to significantly improve the ductility of the weld and HAZ.

Summary of The Invention

Briefly, in accordance with the present invention, an improved method of forming steel components, including but not limited to welded pipe components, is provided. Broadly, the present invention is directed to an improved Post Weld Heat Treatment (PWHT) for hardenable ferrous alloys.

The method of forming a steel member of the present invention includes treating a conventional weld seam formed when welding together two surfaces of a hardenable ferrous alloy. Such an initial weld is formed by applying a heat source, preferably in the form of a conventional welding apparatus, to bring the abutting surfaces to a sufficiently high temperature to melt the ferrous alloy and form a weld. For hardenable ferrous alloys, the weld is then allowed to cool below the onset of martensitic transformation (M)_S) And (3) temperature. The weld may be cooled to ambient temperature. Alternatively, the weld may be cooled to the martensitic transformation of the ferroalloyBeginning (M)_S) Intermediate temperature between temperature and ambient temperature.

After the weld cools below the onset of martensitic transformation (M)_S) After the temperature, the weld is tempered during the post-weld heat treatment. Rapidly heating the weld at a rate of 10 ℃/s or greater above the onset of martensitic transformation (M) of the hardenable ferrous alloy of the weld_S) And (3) temperature. However, the weld is not heated above the lower critical temperature (A) of the hardenable ferrous alloy of the weld_C1). It is important that the weld is rapidly heated at a rate of at least 10 deg.C/s, preferably at a rate of 200 deg.C/s. Although a localized heat source is preferred, a variety of heat sources may be used to apply heat to the weld. Localized heat sources include, but are not limited to, propane or oxyacetylene torches, electrical resistance, electric arc, laser, conduction, irradiation, convection, or high frequency induction methods. The localized heat sources described herein provide heat to the weld and the adjoining region, but do not heat the entire component.

For hardenable ferrous alloys, once the weld is heated above the onset of martensitic transformation (M)_S) Temperature not higher than lower critical temperature (A)_C1) When the weld is tempered, the weld is immediately air cooled and not soaked at the holding temperature for a certain period of time. The air quench is performed at greater than 15 deg.C/minute, but preferably no higher than 200 deg.C/second as may be provided by water cooling.

A wide variety of "hardenable ferrous alloys" that may be used in the practice of the present invention include those steels and alloys that are considered air hardenable. The method of forming a steel member and the post-weld heat treatment method of the present invention are believed to be particularly useful for hardenable martensitic stainless steels, particularly those of the 410, 420 and 440 types. Since different alloys will have different lower critical temperatures, the martensitic transformation starts (M)_S) Temperature and end of martensitic transformation (M)_F) Temperature, and because weld properties will vary depending on the weld design, the tempering rate, tempering finish temperature, and cooling rate will vary.

The method of forming steel components of the invention is also believed to be particularly useful for forming seam welded pipe and tubing components, and for producing circumferential welds (e.g. on gas or liquid tanks).

Accordingly, it is an object of the present invention to provide a method of forming a welded steel member of a hardenable ferrous alloy.

It is a further object of the present invention to provide an improved method of forming a steel member in which heat treatment may be performed during initial production in order to improve the mechanical properties of seam welded joints.

It is yet another object of the present invention to provide a post weld heat treatment system that will improve the ductility of the weld and HAZ without increasing the processing time.

It is a further object of the present invention to provide a method of forming a steel member which is inexpensive and relatively simple to implement.

These and other additional 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 accompanying drawings.

Brief description of the drawings

FIG. 1 is a schematic diagram showing 4 different microstructural zones observed in the Heat Affected Zone (HAZ) of air hardenable steel after welding;

FIG. 2 is a graph of microhardness of a typical weld joint through an air hardenable martensitic stainless steel without prior or post heat treatment;

FIG. 3 is a graph of a conventional tempered post weld heat treatment temperature profile compared to the temperature profile of the present invention;

FIG. 4 is a flow chart depicting the post weld heat treatment process of the present invention;

FIG. 5 is a perspective exploded view of an exemplary clamshell can assembly constructed from a pre-hardened shell prior to flange seam welding;

FIG. 6 is a perspective view of an exemplary clamshell can assembly made up of a pre-hardened shell after flange seam welding;

FIG. 7 is a perspective view of an exemplary clamshell can assembly after bead seam welding, showing partial seam heat treatment with a formed resistive heating coil;

FIG. 8 is a perspective view of an exemplary clamshell can assembly after bead seam welding, showing partial seam heat treatment using a shaped resistive heating coil used;

FIG. 9 is a cross-sectional side view of a typical clamshell can assembly after bead seam welding, showing partial weld heat treatment with heating coils (resistance or induction) located in a holding fixture;

FIG. 10 is a cross-sectional side view of a typical clamshell can assembly after flange seam welding, showing the partial seam heat treatment by passing an electrical current through the shell for the partial seam heat treatment; and

FIG. 11 is a bar graph comparing the maximum strain at break of DIN EN 895 longitudinal tensile specimens of type 410 seam welded samples treated in accordance with the invention, using an average of 62 tests.

Detailed Description

While the present invention is susceptible of embodiment in various forms, as shown in the drawings, there will hereinafter be described preferred embodiments of the invention with the understanding that the present disclosure is to be considered as an exemplification of the invention and is not intended to limit the invention to the specific embodiments illustrated.

The invention includes methods of treating welds or seam welded structures made from high strength steels and other hardenable alloys. The invention is believed to be particularly applicable to alloys that are seam welded to transform into a martensitic weld and HAZ microstructure, and therefore the following description is particularly applicable to such steels. The method of the present invention allows for improved weld ductility and toughness and reduced weld brittleness and susceptibility to hydrogen cold cracking. The present invention allows for improved mechanical strain and deformation of the weld zone (fusion zone and HAZ), thereby eliminating the need for additional post-weld solution heat treatment of the entire component, such as intermediate annealing, subcritical annealing, or stress relief. The localized heat treatment of the present invention eliminates the risk of altering the material properties and microstructure of the base metal, making the present invention suitable for pre-weld thermomechanical treatment of alloys as well as those alloys that are not physically amenable to heat treatment of the entire part.

As outlined in the SAEJ412 specification (GENERAL CHARACTERISTICS AND HEATTREATMENTS OF STEELS), "hardenability or response to heat treatment is probably the most important single criterion for selecting steel. Hardenability is a steel property that determines the distribution and depth of hardness caused by quenching from above the transformation range the term hardenability indicates that the hardness of a material can be increased by suitable treatment, typically involving heating to a suitable austenitising temperature and then cooling at some minimum rate depending on the alloy content. The resulting structure is martensitic if the quenching is complete.

As defined herein, the term "hardenable alloy" refers to a steel grade that is directly hardenable and a ferrous alloy that responds to heat treatment. In addition, the "hardenable alloy" has sufficient carbon content, in combination with other alloying elements, to form a martensitic microstructure in the fusion and HAZ after conventional seam welding. "hardenable alloys" as defined herein have well-defined transition temperatures that depend on the particular chemical composition of the alloy, including: a. the_C3Upper critical temperature, A_C1Lower critical temperature, M_SMartensite Start temperature and M_F-end temperature of martensitic transformation. "hardenable alloys" include those steels and alloys that are considered air hardenable because the natural quench cooling rate associated with seam welding is greater than air quenching. As defined herein, the term "hardenable alloy" excludes those steels and ferrous alloys that are considered "low carbon carburization grades" that respond to heat treatment only by implanting elements into the material surface via a surface hardening process.

Representative hardenable alloys to which the present invention is applicable include, but are not limited to:

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)

SAE 1055、1059、1060、1064、1065、1069、1070、1074、1075、1078、1080、1084、1085、1086、1090、1095、1561(1061)1566(1066)、1572(1072)

SAE 1330、1335、1340

SAE 4037、4047、4130、4135、4137、4140、4142、4145、4150、4161、4340

SAE 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 50100、51100、52100

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

22MnB5

30MnB5

DP600

DP800

DP1000

a preferred method of the invention comprises forming a weld to join two surfaces of hardenable martensitic steel. Allowing the weld to cool below the martensitic transformation of the weldOnset temperature-M_S. The temperature of the weld may or may not be cooled below the martensitic finish temperature or even to room temperature. As shown in fig. 3 and 4, the completed weld is then rapidly heated to a of the weld metal_C1Lower critical temperature (eutectoid temperature), or heating above the start of martensitic transformation (M)_S) A lower intermediate temperature of the temperature and allowing the weld to air cool. In a first embodiment, designated method "A" in FIG. 3 and as shown in phantom on the temperature versus time diagram, the weld is heated to A_C1Temperature not exceeding A_C1-lower critical temperature. This embodiment will allow maximum softening of the weld because all martensite will be subjected to the maximum high temperature tempering. In a second embodiment, designated method "B" in fig. 3 and as shown by the dotted line on the temperature versus time block diagram, heating to an intermediate temperature can be used to improve the toughness of the weld by reducing the brittleness without excessively softening the weld and without excessively reducing the tensile strength of the weld.

Rapid heating of the weld is performed at a rate greater than 10 ℃/s. Preferably, the rapid heating of the weld is performed more rapidly at about 200 ℃/s. Air quenching (no soaking time at the holding temperature) is performed immediately after this rapid heating. The "immediate" transition from rapid heating to air cooling is meant to be understood relatively broadly as including a transition period of seconds or even minutes, which may be incidental to the manufacturing process. However, the "immediate" transition from rapid heating to air cooling is not meant to include isothermal soaking times that allow significant changes to the crystal microstructure of the iron alloy to occur, such as coarsening and recrystallization of the carbon precipitates. The preferred quench rate consistent with air cooling is greater than 15 deg.C/minute but less than 200 deg.C/s.

The weld seam can be heated in its entirety, or section by section (as in the case of continuous seam welding on a rolling mill), with a profiled heat source in the present invention. Heat is applied to the weld using any of a variety of localized heat sources, including but not limited to propane or oxyacetylene torches, electrical resistance, electric arcs, lasers, conduction, irradiation, convection, or high frequency induction. The term "localized" is used herein to describe a heat source that provides heat to a localized area of the component but does not heat the entire component, such as by an oven or oven. In the case of continuous processes, such as seam welded pipe and tubing production, selectively heating the localized weld areas would be the most effective embodiment for larger pipes. Alternatively, it is within the spirit of the invention to heat the entire circumference of the pipe annularly, for example with a helical induction coil or other means. Such annular heating is believed to be more suitable for smaller pipe and tubing diameters. Referring to fig. 5 and 6, in a preferred embodiment, the entire weld may be heated simultaneously, which may be applicable to a variety of configurations including circumferential welds on, for example, hardenable alloy fuel or liquid tanks.

In addition to altering the hardness of the weld zone (i.e., reducing and/or modulating the amount of martensite present in the microstructure of the weld zone), this approach attenuates several other HAZ cracking contributing factors, including:

● allow additional time for hydrogen to diffuse and evolve when the steel is heated in this way. This residual hydrogen is responsible for hydrogen refrigeration cracking in the martensitic microstructure when subjected to applied or residual tensile stress.

● relieve shrinkage strains and stresses in the weld due to the reduced thermal gradient along the length of the weld.

● improve the ductility and toughness of the fusion zone and HAZ.

● temper the weld and any martensite formed in the HAZ.

As shown in fig. 7 and 8, a shaped resistive heating coil may be used to locally heat treat the weld around the entire can perimeter. The heating coil may be applied from the top side, bottom side, or both sides and held in place by being placed in direct contact with or away from the weld surface until the peak temperature is reached. The coil was then removed and the weld was allowed to air cool to room temperature. No protective atmosphere is required; however, if necessary, the treatment may be performed in a non-oxidizing atmosphere.

Referring to fig. 9 and 10, heat may alternatively be applied to the weld using a shaped induction coil or flame or other method. Heating coils may be incorporated into the press die members (fig. 9), which inhibit warping of the weld during processing. In another embodiment of the heat treatment shown in fig. 10, current is passed from one component to another component to resistively heat the weld to a suitable temperature.

Typical automotive structural applications that require seam welding of pre-hardened quenched alloys include chassis components, A, B and C-pillars, roof rails, roof bows, impact beams, and bumpers. The dimensions and amplitude of the resulting body assembly prevent any full component post weld stress relief processing. The post-weld heat treatment method of the present invention is ideally suited to improve the joint properties of the weld state for these or similar types of applications.

In practice, in view of the initial test results for air hardenable martensitic stainless steels, it was found that welds treated according to the first and second embodiments described above had significant ductility and toughness improvements when compared to the as-welded samples. For example, tests were performed with the present invention on seam weld test strips using type 410 stainless steel (UNS41000, SAE51410) of 0.5mm, 1.0mm and 2.0mm thickness. The linear seam weld test fixture was designed to autogenously butt GTAW weld a test strip at a 60 inch length, thereby picking test specimens from the central steady-state portion of the weld. A single-sided linear induction coil is implemented downstream of the GTAW torch body in accordance with the present invention. Direct surface temperature measurements were obtained using a non-contact 3.9 μm wavelength infrared pyrometer.

For type 410 stainless steel, the weld is allowed to cool to about 180 ℃ below the martensite start temperature-M_SAlso below the martensitic transformation end temperature-M_fThey are about 330 ℃ and 230 ℃ respectively. The finished weld was then rapidly heated to about 650 ℃ just below the lower critical temperature-A of about 675 ℃_C1And immediately allowed to air cool. Such as drawing along a weldAs shown in fig. 11 for the maximum elongation in the (longitudinal stretching) direction, the seam welded specimens showed significant improvement in weld and HAZ ductility by tempering according to the method of the present invention. The greatest benefit is seen on thicker test strips because the thicker gauge imposes a greater degree of seam weld joint restraint and therefore higher residual stress, which the heat treatment method of the present invention alleviates. Applying heat to A of the weld seam while processing different alloys_C1The lower critical temperature (eutectoid temperature) or to lower intermediate temperatures may provide different mechanical properties of the pad and the choice will depend on the materials used and the mechanical properties desired.

The present invention is ideally suited for all seam welding processes such as laser welding, resistance seam welding and arc welding. In addition to self-fluxing welds, the present invention is also ideal for treating welds using hardenable weld filler alloys to reduce the brittleness of the fusion zone and HAZ. Upper temperature threshold, A_C1Lower critical temperature (eutectoid temperature) (below which ferrite and carbides are stable) and M_SThe martensitic transformation start temperature depends on the chemical composition of the weld and the base alloy. The natural cooling rate depends on the material thickness, joint geometry, alloy type and environmental conditions.

While several particular forms of the invention have been illustrated and described, it will be apparent that various changes can be made without departing from the spirit and scope of the invention. Accordingly, the invention is not intended to be limited, except as by the following claims. Having thus described the invention, and in order to enable those skilled in the art to understand the invention, reproduce the invention and practice it, and to ascertain the presently preferred embodiments thereof, we claim as follows:

Claims (13)

1. A method of forming a steel member comprising the steps of:

providing a first surface of a hardenable ferrous alloy;

providing a second surface of a hardenable ferrous alloy;

placing the first surface proximate to the second surface;

seam welding the first surface to the second surface by applying a first heat source to the first surface and the second surface at a sufficiently high temperature so that the first surface and the second surface are above their melting points to form a weld;

cooling the weld to below a martensite start temperature of the hardenable ferrous alloy;

after the step of cooling the weld to below the martensitic transformation start temperature of the hardenable ferrous alloy, tempering the weld, the tempering comprising heating the weld at a rate of 10 ℃/sec or greater to above the martensitic transformation start temperature but not above the lower critical temperature of the hardenable ferrous alloy; and

air cooling the weld at a rate of 15 ℃/minute or greater after the step of tempering the weld.

2. The method of forming a steel member of claim 1 wherein said step of tempering said weld is performed at a rate of 10 ℃/sec to 200 ℃/sec and said step of air cooling said weld is performed at a rate of 15 ℃/min to 200 ℃/sec.

3. The method of forming a steel member of claim 1 wherein said step of tempering said weld seam comprises heating said weld seam with a localized heat source.

4. A method of forming a steel member as claimed in claim 3 wherein said step of tempering said weld is performed at a rate of 10 ℃/sec to 200 ℃/sec and said step of air cooling said weld is performed at a rate of 15 ℃/min to 200 ℃/sec.

5. The method of forming a steel structure of claim 1 wherein said hardenable ferrous alloy is an air hardenable martensitic stainless steel having a carbon content equal to or greater than 0.08 wt.%.

6. The method of forming a steel structure of claim 1 wherein each of said hardenable ferrous alloys is a martensitic stainless steel of the type 410, 420 or 440.

7. The method of forming a steel member of claim 1 further comprising roll forming the steel member into a desired shape, the step of roll forming occurring after the steps of welding the first and second surfaces together and tempering the weld.

8. The method of forming a steel member of claim 7 wherein the steel member is a pipe and the first surface defines a first edge of a roll formed strip of ferrous alloy and the second surface defines a second edge of a roll formed strip of ferrous alloy.

9. A method of forming a steel member comprising the steps of:

providing a first surface of a hardenable ferrous alloy;

providing a second surface of a hardenable ferrous alloy;

placing the first surface proximate to the second surface;

seam welding the first surface to the second surface by applying a first heat source to the first surface and the second surface at a sufficiently high temperature so that the first surface and the second surface are above their melting points to form a weld;

cooling the weld to below a martensite start temperature of the hardenable ferrous alloy;

tempering the weld after the step of cooling the weld to below a martensitic transformation start temperature of the hardenable ferrous alloy, the tempering comprising heating the weld with a localized heat source at a rate of 10 ℃/sec to 200 ℃/sec to above the martensitic transformation start temperature but not above a lower critical temperature of the hardenable ferrous alloy; and

immediately air cooling the weld at a rate of 15 ℃/minute to 200 ℃/second after the step of tempering the weld.

10. The method of forming a steel structure of claim 9 wherein said hardenable ferrous alloy is an air hardenable martensitic stainless steel having a carbon content equal to or greater than 0.08 wt.%.

11. A method of forming a steel structure as claimed in claim 9 wherein each said hardenable ferrous alloy is a martensitic stainless steel of the type 410, 420 or 440.

12. The method of forming a steel member of claim 9 further comprising roll forming the steel member into a desired shape, the step of roll forming occurring after the steps of welding the first and second surfaces together and tempering the weld.

13. The method of forming a steel member of claim 12 wherein the steel member is a pipe and the first surface defines a first edge of a roll formed strip of ferrous alloy and the second surface defines a second edge of a roll formed strip of ferrous alloy.