INTEGRATED WELDING AND THERMAL PROCESSING JOINING METHOD FOR CREEP STRENGTH ENHANCED FERRITIC STEELS

Abstract

An integrated welding and thermal processing method includes heating adjoining surfaces, at least one of which is a creep strength enhanced ferritic (CSEF) steel alloy, to a sufficiently high temperature above their melting points to form a weld. The weld is allowed cool below the martensitic start temperature of one or both CSEF alloys. Thereafter, a supplemental heat source tempers the CSEF alloys by reheating the weld area at a rate of 10° C. per second or greater to above the CSEF alloys’ martensitic start temperatures, but not above the austenitization temperature of the CSEF alloys. After the weld’s heat affected zone is maintained at a temperature between the CSEF alloys’ martensitic finish temperature and martensitic start temperature, the weld is allowed to cool at a rate of 15° C. per minute or greater.

Claims

1. A method of forming a weld comprising the steps of: providing a first surface of a creep resistant alloy having a carbon content equal or greater than 0.07% by weight, having a chromium content of 8.0% - 12% by weight, and having a molybdenum (Mo) or vanadium (V) or tungsten (W) or tantalum (Ta) content or combination thereof of 0.85% or greater by weight; providing a second surface of a weldable alloy; positioning said first surface adjacent to said second surface; welding said first surface to said second surface by applying a first heat source to said first surface and said second surface to heat said first surface and second surface to a sufficiently high temperature above their melting points to form a weld; allowing said weld to cool to below the martensitic start temperature of said first surface creep resistant alloy; tempering said weld subsequent to said step of allowing said weld to cool to below said first surface’s creep resistant alloy’s martensitic start temperature but before said weld cools below said first surface’s creep resistant alloy’s martensitic finish temperature, said tempering including heating said weld at a rate of 10° C. per second or greater to above said first surface’s creep resistant alloy’s martensitic start temperature but to not above the austenitization temperature of said first surface’s creep resistant alloy; and allowing said weld to cool at a rate of 15° C. per minute or greater after said step of tempering said weld.

2. The method of forming a weld of claim 1 wherein: said second surface of a creep resistant alloy having a carbon content equal or greater than 0.07% by weight, having a chromium content of 8.0% - 12% by weight, and having a molybdenum (Mo) or vanadium (V) or tungsten (W) or tantalum (Ta) content or combination thereof of 0.85% or greater by weight; said step of allowing said weld to cool includes allowing said weld to cool to below the martensitic start temperatures for both of said creep resistant alloys; and tempering said weld includes allowing said weld to cool to below the martensitic start temperature for both of said creep resistant alloys but before said weld cools below said martensitic finish temperatures of both said creep resistant alloys, and tempering including heating said weld at a rate of 10° C. per second or greater to above both said first and second surfaces’ creep resistant alloys’ martensitic start temperatures but to not above the austenitization temperature for both of said creep resistant alloys.

3. The method of forming a weld of claim 1 wherein said step of tempering including heating said weld at a rate of 10° C. per second or greater includes induction heating.

4. The method of forming a weld of claim 1 wherein said step of tempering including heating said weld at a rate of 10° C. per second or greater includes use of a laser.

5. The method of forming a weld of claim 1 wherein said first surface of a creep resistant alloy is Grade 91 or Grade 92 creep strength enhanced ferritic (CSEF) steel.

6. The method of forming a weld of claim 2 wherein said first surface of a creep resistant alloy and said second surface of a creep resistant alloy are a Grade 91 or Grade 92 creep strength enhanced ferritic (CSEF) steel.

7. The method of forming a weld of claim 1 wherein said first surface of a creep resistant alloy is reduced activation ferritic-martensitic (RAFM) steel.

8. The method of forming a weld of claim 2 wherein said first surface of a creep resistant alloy and said second surface of a creep resistant alloy are reduced activation ferritic-martensitic (RAFM) steels.

9. The method of forming a weld of claim 2 wherein said first surface’s creep resistant alloy and said second surface’s creep resistant alloy are the same creep resistant alloy.

10. The method of forming a weld of claim 2 wherein said first surface, said second surface and said weld form the longitudinal seam of a pipe.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] FIG. 1 is a plot of the temperature profiles obtained using the integrated welding and thermal processing method applied to Grade 91 CSEF steel;

[0020] FIG. 2 is flow chart illustrating the integrated welding and thermal processing method; and

[0021] FIG. 3 is a plot of the hardness profiles obtained using conventional weld processing and PWHT as compared to the integrated welding and thermal processing method as applied to Grade 91 CSEF steel.

DETAILED DESCRIPTION OF THE INVENTION

[0022] The present invention addresses the aforementioned disadvantages by providing an integrated welding and thermal processing method for creep resistant alloys. While the integrated welding and thermal processing method is susceptible of embodiment in various forms, as shown in the drawings, hereinafter will be described the presently preferred embodiments of the invention with the understanding that the present disclosure is to be considered as an exemplification of the invention, and it is not intended to limit the invention to the specific embodiments illustrated.

[0023] Although the following description refers specifically to creep resistant alloys including CSEF steels and RAFM alloys, the present invention is applicable to any hardenable ferrous alloy having a carbon content equal or greater than 0.07% by weight, and having a chromium content of 8.0% - 12% by weight; and having molybdenum (Mo) or vanadium (V) or tungsten (W) or tantalum (Ta) or combinations thereof content of 0.85% or greater by weight, and which it is desired to have a weldment with reduced hardness and increased toughness, without subjecting the joint to a separate PWHT. Typical CSEF grades include modifications of ASTM/ASME SA-213 T9 (Grade 9) 9Cr-1Mo alloys including, but not limited to: Grade 91, Grade 92, Grade 911, Grade 122, Grade 23, VM12HC, and specialty RAFM alloys including, but not limited to: Eurofer97, F82H, JLF-1, CLAM, and oxide dispersion-strengthened (ODS) alloys, also known as nanostructured ferritic alloys (NFA).

[0024] As previously described, common creep resistant alloy composition is controlled under ASTM A387 / ASME SA 387 specifications for plate, and ASTM A335 /ASTM 691 / ASME SA213 specifications for tube and pipe; the representative Grade 91 creep resistant alloy nominal composition contains primarily Fe, with Cr 8.00-9.50%; Mo 0.85-1.05%; C 0.08-0.12%; V 0.18-0.25%; and Nb 0.06-0.10% by weight, with minor additions of Mn, Si, and N, and impurity limits for P, S, Ni, Al, Ti, and Zr. Typical Grade 92 composition contains primarily Fe, with Cr 8.50-9.50%; Mo 0.30-0.60%; C 0.07-0.13%; V 0.15-0.25%; W 1.50-2.00%; and Nb 0.04-0.09% by weight, with minor additions of Mn, Si, B and N, and impurity limits for P, S, Ni, Al, Ti, and Zr.

[0025] RAFM creep resistant alloy compositions are modifications of these of ferritic-martensitic 8-12% Cr-MoVNb steels mainly by exchanging Mo, Nb and Ni with W and Ta in order to obtain low activation capability. For example, Eurofer97 composition targets by weight are Cr 8.50-9.50%; C 0.09-0.12%; W 1.00-1.20%; V 0.15-0.25%; Mn 0.20-0.60%; Ta 0.10-0.14%; N 0.015-0.045%, with impurity limits for Nb, Mo, Ni, Cu, Al, Ti, Si, Co, P, S, B and O. Similarly, F82H, another RAFM creep resistant alloy, has target composition by weight of Cr 8.00%; C 0.10%; W 2.00%; V 0.20%; Mn 0.30%;Ta 0.04%; N 0.010%, with impurity limits for Nb, Mo, Ni, Cu, Al, Ti, Si, Co, P, S, B and O.

[0026] With reference to FIGS. 1- 3, the integrated welding and thermal process is accomplished with the addition of a secondary heat source to provide supplemental heating immediately following weld pool solidification. Certain embodiments of the present invention include, but are not limited to, a gas tungsten arc welding (GTAW) primary heat source closely coupled to a single sided induction coil with infrared pyrometer feedback. Other primary and secondary heat sources may include laser, gas metal arc welding (GMAW), plasma arc welding (PAW) resistance, electron beam, or solid-state (i.e., friction stir welding). Weld processes may be autogenous or use matching or dissimilar filler wire composition. The coupled design with adjacent primary-secondary heat sources allows for independent control of the welding operation (e.g., weld penetration, fusion zone width, deposition rate, travel speed) and the in-situ weld cooling profile. Linear and curved joints on materials of varying thickness are possible with such an arrangement, including but not limited to multi-pass weldments on thick sections.

[0027] As demonstrated, a solid state induction heater can offer digital control of induction frequency, penetration depth and width, and overall power when used as a secondary heat source. Control of these parameters, and location of the secondary heat source with respect to the primary weld torch, can define various cooling-control profiles. Immediate temperature feedback can be provided by non-contact means such as infrared pyrometers measuring weld seam surface temperature, among other non-contact and contact-based methods. On some weldments, thermocouples can be affixed to expected heat affected zones (before welding) to control and monitor temperature profiles as function of time.

[0028] With reference to FIGS. 1 and 2, the intent of the present invention is to provide inline weld cooling control and reheating up to the lower critical transformation temperature (Aci) before the weld cools to ambient temperature. As best illustrated in FIG. 1, the greatest benefits are observed by immediately reheating the weld seam near the lower critical transformation (Aci) temperature after weld solidification, with heating parameters dependent on weld travel speed and thickness. This lower critical temperature is a function of the particular alloy composition. As a point of reference, the nominal Aci temperature for “9Cr-1Mo” CSEF steels such as Grade 91 with nominal 9 wt.% chromium content is approximately 810° C. (1490° F.). In certain instances, rapid heating of the freshly created weld in excess of 200° C./s with the secondary heat source can shift the Aci temperature upward when compared with quasi-equilibrium values. This can be exploited in the present invention thereby increasing the process window parameters and weld softening achieved while avoiding further austenitic transformation. To illustrate, industry-recognized standards for conventional PWHT of Grade 91 steel currently dictate an ideal temperature of 760° C. (1400° F.) for 2 hours, maintaining these precise conditions with controls typically calibrated for ± 5° C. (9 ±°F) accuracy or better. Extreme care is taken to avoid exceeding this temperature threshold, which is approximately 50° C. (90° F.) below the Aci temperature. In contrast, the integrated weld and thermal processing of the present invention is shown to have significant weld softening effects with secondary heat source induced temperatures of 810 ± 20° C. (1490 ±°F), a much larger process window of 20° C. (36° F.) below and above the Aci temperature. This secondary heat is applied before the weld cools to room temperature. In typical cases the weld cools very rapidly with cooling rates during solidification on the order of 500-1000° C./s (900-1800° F./s), and secondary heat is applied when the freshly created weld is in the 300° C. (572° F.) temperature range. Once tempered to a desired reduction in weld brittleness and elimination of hydrogen-induced cracking, the weld is allowed to cool at a rate of 15° C. per minute or greater.

[0029] This integrated welding method can take the physical form of a novel welding head with an integrated secondary heat source, or discrete components for weld creation and secondary heating. It may be applied to manual welding devices or robotic welding end-effectors. Such precedent exists with hybrid welding modes for enhanced deposition like GMAW/GTAW double arc welding, laser assisted GTAW, or its complement, GTAW assisted laser welding.

[0030] This method can solve many of the historical difficulties associated with welding of CSEF steel - and when optimized can promote transformation of the weld and heat affected zone into tempered martensite and very fine carbides. Thus, reducing CSEF weld brittleness and eliminating hydrogen-induced cracking, while improving ductility and toughness, without requiring weld pool dilution using non-matching weld filler alloys, or lengthy off-line pre-and/or PWHT.

[0031] Referring to FIG. 3, the integrated welding and thermal processing method was conducted using an experimental weld test sled which allowed for controlled testing of the process on CSEF steel strips using a GTAW weld torch and secondary heating and control using non-contact induction heating equipment. Experiments were carried out on Grade 91 CSEF steel on 3 mm thick square edge butt-joint autogenous weld configurations. The experiment featured Type-K thermocouples affixed to the CSEF specimens prior to welding so that temperature profiles may be monitored. Still referring to FIG. 3, the measured CSEF weld microhardness is plotted for welds created by the aforementioned experiments. Microhardness profiles of conventionally processed and integrated welding and thermal processed welds, Grade 91 CSEF specimens are plotted. “As-welded” refers to conventional welds; “PWHT” denotes conventional “As-welded” specimens subjected to an off-line, 1 hour 732° C. (1350° F.) heat treatment; and “Integrated weld + thermal” correspond to optimal integrated welding and thermal processing parameters. For comparison, results are included from conventional weld processing and conventional welding with an off-line PWHT. CSEF steels exhibit high hardness in the weld fusion zone and heat affected zone upon welding with conventional methods, often exceeding 400 HV. Conventional off-line PWHT is known to reduce hardness in these zones to that near the base metal hardness, 200-250 HV in this case for Grade 91. Optimum integrated welding and thermal processing results in a 125 HV reduction in weld fusion and heat affected zone hardness, nearly approaching that of a conventional PWHT.

[0032] Furthermore, the integrated weld and thermal processing technology is ideally suited for RAFM structures within fusion reactors. It is also applicable to all CSEF steels and there is a clear need for improved joining and welding methods for these materials. CSEF alloys are favored by the power generation industry for their cost-effective pressure boundary performance at elevated temperatures and are used in a variety of power generation and high temperature process applications including heat exchangers such as heat recovery steam generators (HRSGs), superheaters, boilers, reactors, pressure vessels and piping. Typical HRSGs are large, complex pipe-tube assemblies with thousands of weldments. Grade 91 and Grade 92 steels are favored for high temperature regions. Sizes can exceed 100 MW, with millions of square feet of heat exchange area and capital costs in tens of millions of dollars. Smaller systems (<5 MW) do exist, and find wide applications across commercial, refining, centralized heating, power generation and petrochemical industry. As large fabricated structures, growth of HRSGs are somewhat limited due to the high cost of construction, transportation, siting, and regulatory concerns. Additional restrictions for PWHT during construction or repair can be limiting - hence the motivation for new shop and field joining technologies such as the invention described here. However, the real cost is after commissioning - a typical plant may experience plant shutdowns for repair work; half of unexpected shutdowns are caused by boiler tubes. Shutdowns for cracked or failing welds are not uncommon, particularly as more plants using CSEF steels are gaining creep-relevant operational experience past their first decade. The integrated weld and thermal processing method described here may also be used for repair work on similar large structures, providing a cost-time-performance advantage to conventional weld and PWHT repair processing.

[0033] Accordingly, it will be apparent that various modifications can be made without departing from the spirit and scope of the invention. Therefore, having described my invention in such terms such as to enable a person skilled in the art to understand the invention, recreate the invention and practice it, and having presently identified the presently preferred embodiment thereof, we claim: