GB2079659A - Method for heat treating metal - Google Patents

Abstract

A method of relieving stress in microstructure welds and in the heat- affected zone in the adjacent base metal in low alloy steel, comprises induction heating in a second post-weld heat treatment. The heated area is monitored with a radiation pyrometer to ensure precise temperature control of the procedure. The method is particularly applicable to the heat treatment of selected small areas of a metal article without heating the whole article.

Description

SPECIFICATION method for heat treating metal The present invention relates to relieving stress in the microstructure of metal by induction heating, and more particularly to the stress-relief of welds and surrounding areas of base metal in low alloy steels employed in hostile subsurface environments such as are encountered in the petroleum industry.

In many areas of the world, hydrogen sulfide, commonly called "sour gas" is encountered where a petroleum well is drilled. The presence of sour gas in a well dictates a choice of materials which will not be subject to sulfide stress cracking, which may be described as the brittle failure of a metal under the combined action of tensile stress and corrosion in the presence of hydrogen sulfide in an aqueous environment.

The National Association of Corrosion Engineers (NACE) has published a Material Requirement deal ing with "Sulfide Stress Cracking Resistant Metallic Material For Oil Field Equipment," NACE Standard MR-01-75 (1980 Revision), which standard is em ployed as a materials guideline by the petroleum industry when dealing with sour gas. The standard sets forth various requirements for the acceptable performance in sour gas of different ferrous and non-ferrous metals, fabrication, bolting, platings and coatings thereof, as well as requirements for various specific components and devices used in well drill ing, testing, production and servicing. Low alloy steels, being those containing less than about five percent (5%) total alloying elements, are acceptable materials for a sour gas environment provided they meet certain requirements set forth in NACE Stan dard MR-01-75.

In general, if these steels contain less than one percent (1%) nickel (Ni), they are acceptable provided they are subjected to certain thermal treat ments to alter the microstructure of the steel. It is emphasized by the NACE Standard that there is a definite correlation between sulfide stress cracking, heat treatment and hardness in metals, which corre lation has been proven by extensive laboratory and field data. Hardness is in part a function of the stress present in the microstructure; the resistance of the metal to sulfide stress cracking is enhanced by the lowering of hardness by stress relief in the microstructure through heat treatment. As hardness in an accurate, nondestructively generated test para meter, it is extensively used to monitor materials performance.The Rockwell "C" Hardness Scale (HRC) as used in the NACE Standard is the primary basis for determination of an acceptable hardness, for sour gas equipment, although it should be understood that other hardness scales may be employed using suitable conversion factors for correlation purposes. As a rule, a hardness of HRC 22 is the maximum hardness allowable by NACE for low-alloy steels and welds thereon in a sour gas environment. A hardness of up to HRC 26 may be tolerated for certain tubular goods, but only if adequate performance is verified with a sulfide stress cracking test, a procedure necessitating additional expense. Therefore, it is desirable to obtain a hardness of HRC 22, it being understood that this figure may be the average of several tests, as long as the maximum HRC of a specimen does not exceed 23 or 24.

In many instances, a hardness of HRC 22 maximum for steel and welds thereon can be obtained by heat treating the material or apparatus in question in a furnace, but in other instances this is impractical, such as where the apparatus is assembled and then welded, the assembly including components such as elastomeric seals, which are destroyed by the high furnace temperatures. Likewise, even in the absence of seals, there may be finished surfaces in the apparatus which would be damaged by prolonged exposure to high temperatures necessitated by furnace treatment. Thus, there presently exists a problem in the production of some material or assembly of apparatus to the NACE Standard.

We have now devised a method of thermally treating a metal, particularly low alloy steel and welds thereon, by which the heating may be applied only to the specific area to be stress relieved. By employing such a method, seals and other destructible nonmetallic elements as well as finished surfaces in relatively close proximity to the heat treated areas can remain at low enough temperature to ensure their integrity.

According to the invention, there is provided a method of heat treating metal, comprising: subjecting said metal to induction heating for a sufficient time at a subcritical temperature to achieve a hardness of substantially 23 or less on the Rockwell "C" hardness scale.

The invention also includes a method of achieving a hardness of no more than 23 on the Rockwell "C" hardness scale in the heat-affected zone in base metal surrounding a weld, comprising: preheating the base metal prior to welding; maintaining a sufficient interpass temperature to retard the cooling rate of the weld area to a sufficient degree; welding while maintaining said interpasstemperature; cooling said weld and base metal below Ms; first postheating said weld and base metal; cooling said weld and base metal; second postheating by induction heating said weld and adjacent base metal.

The invention further includes a method of heattreating steel, comprising: subjecting said steel to induction heating for a sufficient time at a subcritical temperature to achieve a microstructure hardness on the Rockwell "C" scale of no less than substantially 18, and no greater than substantially 23.

The invention also provides a method of achieving a microstructure hardness of at least substantially 18, and no more than substantially 23 on the Rockwell "C" scale in the heat affected zone surrounding a weld in AISI Grade 4140 low alloy steel, comprising: induction heating said heat affected zone at a subcritical temperature for at least substantially 350 seconds.

The invention further provides a method of induction heating a tubular workpiece to affect a postweld heat treatment, comprising: positioning said workpiece between inner and outer induction coils; rotating said workpiece about its longitudinal axis while generating heat with said induction coils; sensing the temperature in said workpiece at the weld and heat-affected zone of base metal surrounding said weld with a radiation pyrometer through an infrared sensor; and controlling said workpiece temperature at a predetermined level in response to the output of said radiation pyrometer.

In order that the invention may be more fully understood, reference is made to the accompanying drawings, in which: Figure 1 is a schematic sectional view of one form of apparatus which may be used in the practice of the present invention, with a metal article to be selectively heat treated oriented in position; Figure 2 is a drawing depicting the micro-structure of the heat affected zone of the base metal adjacent a fusion weld, such as could be found on the article to be treated in Figure 1, prior to heat treatment; and Figure 3 is a drawing depicting the microstructure of the heat affected zone of Figure 2 after heat treatment by the method of the present invention.

Stress-relief of microstructures in metals by heating in a furnace is common in many industries.

Generally, the heating takes place over a relatively long period of time, measured in hours. Induction heating, on the other hand, is usually effected in minutes or seconds. Certain variables result in different times and temperatures for obtaining equivalent results from the two types of heating, the relationship of which has been expressed in the following equation: TW (C + log t) = TF (C + log tF) where TF and tF are the furnace temperature (in degrees Rankine) and time, respectively, known to produce a given hardness in a metal, and T1 and tl are the equivalent temperature (in degrees Rankine) and time needed to produce that hardness with induction heating. C is a constant which may be empirically determined for a given metal, and is approximately 15 for steels with 0.25 to 0.50% carbon content.The figures obtained from the above equation for time and temperature to be employed in induction heating of a specific metal are, of course, further refined by empirical testing.

Figure 1 schematically depicts an induction heating apparatus designated generally at 10. Induction heating apparatus 10 comprises generator and load coil transformer 12, to which inner induction coils 14 and outer induction coils 16 are connected. A suitable generator is a Lepel 100kw generator, Model T-100-3kcTL. Precise temperature control at the workpiece 30 is achieved by use of radiation pyrometerithree mode proportional controller 18, which controls the generator, varying the output thereof in response to the input of infrared sensor 20, which senses the temperature on the workpiece 30. A suitable pyrometer/controller is the IRCON, model 6-22F1 5-01-000-1/620. A portion of tubular workpiece 30 is shown disposed between the inner induction coils 14 and the outer induction coils 16.The two coils are employed to avoid a temperature gradient in the workpiece 30, and further temperature uniformity is achieved by mounting the workpiece 30 on a rotating jig (not shown), whereby the workpiece 30;is rotated about its longitudinal axis during the induction heating procedure.

For the sake of illustration, and not by way of limitation, the workpiece 30 shown is a portion of a Halliburton Services F.O. Multiple Stage Cementer, described on page 3347 of Halliburton Services Sales and Service Catalog Number 40. This type of workpiece poses several impediments to furnace stress-relief, namely the presence of elastomeric seals 32 and of finished surface 34. Fusion weld 36 is to be stress-relieved with no damage to the surface 34 or elastomeric seals 32.

The F.O. Multiple Stage Cementer comprises metallic parts of AISI (American Iron and Steel Institute) Grade 4140 low alloy steel. The elastomeric seals 32 have a maximum temperature destruction tolerance of 325"F (163"C). Finished surface 34 will begin to scale at 1050"F (565"C). As it is necessary for the operation of the F.O. Multiple Stage Cementer that sleeve 36 slide within finished surface 34, scaling on surface 34 may result in an inoperative tool. If there were no seals or finished surfaces, the workpiece could be stress-relieved by furnace heating for a prolonged period at 1 3000F (705"C). However, as the assembled Cementer does include these items, a furnace treatment is impossible.The following procedure, unlike a furnace heat treatment, will result in an assembly and welds thereon with the desired hardness characteristics.

Priorto assembly, the metallic parts of the F.O.

Multiple Stage Cementer, comprising fine grain seamless, hot finished and normalized AISI Grade 4140 low alloy steel, are subjected to stress-relief heating in a furnace at 1300"F (705"C) for several hours. Subsequently, the desired finished surface 34 is machined, and the tool is assembled with elastomeric seals 32.At that point, fusion weld 86 is made by, for example, a submerged arc welding process (SAW), with a preheat at the weld point of 700"F (361"C), an interpass temperature of 650"F (344"C) during the welding process, followed by cooling to below the M5 temperature, the temperature at which martensite begins to form, which is approximately 500"F (260"C). The interpass temperature lowers the cooling rate of the metal to an acceptable level, minimizing residual stresses in the microstructure.Subsequently, the weld is subjected to a first postheat at 800"F (427-427OC). A low alloy steel electrode comprising less than 1% nickel (Ni) is employed in the SAW process, being an ASME (American Society of Mechanical Engineers) SF A5.28 ER80S-D2. Such electrodes are available from Union Carbide Corporation Linde Division as Linde 83, or Page Division of Acco as Page 18. The weld flux employed may be neutral Fxxx, classes perAWS (American Welding Society) A5.17-76 or A5.23-77. A suitable flux is Lincoln Electric 880 Flux.

Subsequent to the first postheat, the workpiece80 is cooled and placed on a jig, which orients the workpiece 30 between inner induction coils 14 and outer induction coils 16 of induction heating apparatus 10. The workpiece 30 is slowly rotated on the jig about its axis between the two coils during which rotation the weld 36 is then subjected to a second postheat at substantially 1340"F (726-727"C) for 900 seconds. The temperature is measured by sensor 20 at the metal surface of the workpiece 30. Radiation ,syrometer/three mode proportional controller 18 maintains this temperature in an accurate manner by controlling the output of generator and load coil transformer 12 to induction coils 14 and 16.As 1340 F (726-727"C) is close to the critical temperature of 1354"F (735"C) for AISI Grade 4140 low alloy steel, it is imperative that the stress-relief temperature be closely monitored to avoid a supercritical temperature in the metal.

Actual empirical testing of AISI Grade 4140 speci mens welded and subjected to a second postheat according to these specifications has shown the hardness of the heat-affected zone (HAZ) in the base metal adjacent the weld to under HRC 23 after stress-relief. Test specimens were stressed to 100% of their transverse tensile yield strength, with the maximum stress located across the fusion zone of the weld, and subjected to a 5% sour brine corrodent at atmospheric pressure at 65"F (18.5 C) temperature for 30 days. All specimens resisted sulfide stress cracking for the 30 day period.

Referring to Figures 2 and 3 of the drawings, which are representative of the microstructure of the HAZ of base metal adjacent a weld at 200 x magnification on an F.O. Multiple Stage Cementer, it can be seen in Figure 2 that there was a moderately stressed microstructure in the HAZ after the first post-weld heating, comprising martensite as well as bainite, the latter of which appears as groups of needle-like structures. Figure 3, depicting the HAZ adjacent the weldment after the second postheat, shows a much more refined grain structure in the HAZ, with attendant lowered stress level. The HAZ hardness was reduced from HRC 29 measured in Figure 2, to HRC 20 measured in Figure 3. During the stress relief-process, the maximum temperature at 9.5 inches from the weld along the outer case of the F.O.

Multiple Stage Cementer was 295"F (145"C) below the destruction temperature of the elastomeric seals 32, and well below scaling temperature for finished surface 34.

It should be noted that the preheat employed in the SAW process contributes to the success of the subsequent second postheat as it enhances the formation of bainite (designated as B in Figure 2) along with the martensite of the microstructure, thus reducing the initial hardness of the HAZ.

It may also be noted that the temperatures and times given in the above illustration are variable to a certain extent for the desired results. For example (again assuming AISI Grade 4140 low alloy steel), a preheat as low as substantially 675"F (357"C) or as high as substantially 800"F (426-427 C) may be employed, with acceptable results. The higher pre heat temperature of 800"F (426-427 C) will result in less martensite formation and a softer post-weld microstructure, but the lower temperatures will produce acceptable results.Likewise, a first postheat of substantially 775"F (413"C) to substantially 900"F (482"C) may be utilized. The first postheating is the least critical of the heating steps, being used to drive off montonic hydrogen from the weldment. Similarly, the interpass temperature of 650"F (343-344"C) is an approximation, the important consideration being the reduction of the cooling rate at the weld area to an acceptable area.

It should also be understood that the critical temperative given for AISI Grade 4140 low alloy steel may vary appreciably from 1354"F (735 C), depending upon the exact chemical composition of the bar stock obtained. The critical temperature may range from 1340"F to 1395"F (726 to 757"C), thus allowing some minor variation of the induction heating postheat temperature. For example, a minimum time of 350 seconds at substantially 1335"F (724"C) may be employed, and acceptable results obtained.

The second postheat temperature of 1340"F (726.5"C) may also be modified downward somewhat, for example to substantially 13100F (710"C) for a time of substantially 900 seconds. Below this temperature, the time for heating becomes too long from an economic standpoint. Furthermore, it is desirable to maintain hardness in the range of HRC 18-22, to preserve the mechanical properties of the weld material and surrounding base metal in the HAZ, which too long a heating time may prevent. Equipment used in sour gas in the petroleum industry must meet the API (American Petroleum Institute) L-80 tensile requirements, as well as the L-80 hardness requirement of HRC 23 maximum. The minimum acceptable tensile yield strength is 80,000 psi.Reduction of hardness to below substantially 18 HRC will result in failure to meet this requirement.

While it may not be necessaryfortheweidment itself to meet this requirement, if the hardness in the HAZ is reduced below substantially 18 HRC, the base metal in the HAZ will fail at too low a stress.

It should also be noted that the use of temperatures in the second postheat below substantially 1310"F (710"C) in the instances where seals, finished surfaces or other destructible elements are present, may result in damage to those elements due to the necessarily longer time exposure at what must still be a high temperature, to achieve the desired results.

While reference has been made to the specific example of treating AISI Grade 4140 low alloy steel, it should be understood that the present invention is not so limited. Other low alloy steels, as well as some carbon steels, are susceptible to treatment in a similar fashion.

It is thus apparent that the present invention comprises a new and different method for heat treating metals which will be subject to a sour gas environment when furnace heating is unworkable.

Additionally, the procedure may be accomplished in a very short period of time, with high quality control and uniformity. The method, of course, while illustrated with respect to welds and surrounding HAZ in base metal, is not so limited; rather it is applicable wherever precise, localized heat treating to relieve stress in the microstructure of metals is desired. It will be readily apparent to one of ordinary skill in the art that modifications, additions and substitutions to the disclosed method can be made, the invention being limited only by the spirit and scope of the appended claims.

Claims (22)

1. A method of heat treating metal, comprising: subjecting said metal to induction heating for a sufficient time at a subcritical temperature to achieve a hardness of substantially 23 or less on the Rockwell "C" hardness scale.

2. A method according to claim 1, wherein said metal is steel.

3. A method according to claim 2, wherein said time is at least 350 seconds.

4. A method according to claim 2, wherein said time is no more than 900 seconds.

5. A method according to claim 2, 3 or 4, wherein said subcritical temperature is at least710 C.

6. A method according to claim 3, wherein said temperature is at least 724"C.

7. A method according to claim 4, wherein said temperature is 726 to 727"C.

8. A method of achieving a hardness of no more than 23 on the Rockwell "C" hardness scale in the heat-affected zone in base metal surrounding a weld, comprising: preheating the base metal prior to welding; maintaining a sufficient interpass temperature to retard the cooling rate of the weld area to a sufficient degree; welding while maintaining said interpass temperature; cooling said weld and base metal below Ms; first postheating said weld and base metal; cooling said weld and base metal; second postheating by induction heating said weld and adjacent base metal.

9. A method according to claim 8, wherein said metal is AISI Grade 4140 low alloy steel, and: said preheating is conducted at a temperature in the range of 357 to 427DC; said interpass temperature is substantially 343 to 344"C; said welding is fusion welding; said first postheating is conducted at a temperature in the range 413 to 482"C; and said second postheating is conducted at a temperature of at least 710 C.

10. A method according to claim 8 or 9, wherein said induction heating is conducted at a temperature of at least substantially 724"C for at least substantially 350 seconds.

11. A method according to claim 8 or 9, wherein said induction heating is conducted at a temperature of at least substantially 710"C for no more than substantially 900 seconds.

12. A method according to claim 8 or 9, wherein said induction heating is conducted at a temperature of substantially 726 to 7270C for a time of substantially 900 seconds or less.

13. A method of heat-treating steel, comprising: subjecting said steel to induction heating for a sufficient time at a subcritical temperature to achieve a microstructure hardness on the Rockwell "C" scale of no less than substantially 18, and no greater than substantially 23.

14. A method of achieving a microstructure hardness of at least substantially 18, and no more than substantially 23 on the Rockwell "C" scale in the heat affected zone surrounding a weld in AISI Grade 4140 low alloy steel, comprising: induction heating said heat affected zone at a subcritical temperature for at least substantially 350 seconds.

15. Amethod according to claim 14, wherein said subcritical temperature is at least substantially; 724'C.

16. A method according to claim 14 or 15 where- in said time of at least substantially 350 seconds is no more than substantially 900 seconds.

17. A method according to claim 14, wherein said subcritical temperature is at least substantially 710"C, and said time is substantially 900 seconds.

18. A method according to claim 14, wherein said subcritical temperature is substantially 726 to 727"C and said time is substantially 900 seconds or less.

19. A method A method of induction heating a tubularworkpieceto affect a post-weld heattreatment, comprising: positioning said workpiece between inner and outer induction coils; rotating said workpiece about its longitudinal axis while generating heat with said induction coils; sensing the temperature in said workpiece at the weld and heat-affected zone of base metal surrounding said weld with a radiation pyrometerthrough an infrared sensor; and controlling said workpiece temperature at a predetermined level in response to the output of said radiation pyrometer.

20. A method according to claim 19, wherein said predetermined level is below the critical temperature of said base metal of said workpiece.

21. A method of heating metal substantially as herein described with reference to Figure 1 of the accompanying drawings.

22. Metal which has been heat treated by the method of any preceding claim.