CA1131104A

CA1131104A - Process for treating weldments

Info

Publication number: CA1131104A
Application number: CA349,104A
Authority: CA
Inventors: Raj K Malik
Original assignee: US Department of Energy
Current assignee: US Department of Energy
Priority date: 1979-04-25
Filing date: 1980-04-02
Publication date: 1982-09-07
Also published as: DE3015638A1; FR2455087A1; GB2048146B; GB2048146A; JPS55145124A

Abstract

A PROCESS FOR TREATING WELDMENTS
ABSTRACT OF THE DISCLOSURE
me tensile ducttility and impact strength. of weldments of nickel-based and stainless steel alloys are improved to that of the unaffected base metal by subjecting the weldments to an elevated temperature at an isostatic pressure for a period of time sufficient to render the material in the weld more homo-geneous.

Description

~131104 A PROCESS FOR TREATING WELDMENTS

~ACKGROU~1D OF TH~ II~E~1TION
The nickel-based alloys and stainless steel alloys are high-strength corrosion-resistant materials having a wide tem-perature range of ueilization. Because of their many desirable qualities, these materials are widely used in the construction of nuclear reactors. Although both of these types of alloys are readily weldable by any of the arc welding processes such as gas tunaston arc, weldments of these materials are subject to microcracking, segregation of carbides, and formation of laves or embrittling phases, and reduction in ductility and ductilily-dependent properties such as impact strength and fracture tough-ness. These effects are most prominent in the region of the base metal which has undergone a metallurgical change as a result of exposure to welding heat called the heat-affected zone (HAZ). The metal in this zone has been heated and cooled through a range of temperatures sreat enough to cause physical changes in the structure of the metal wlth consecuent changes in physical properties. ~ith steels, a hard zone will be pro-duced depending upon the degree to which the particular steels are hardened. This is accompanied by a loss of ductility which will decrease the resistance to shock or fatigue. Brittleness ~ay also exist through the development of a coarse grain struc-ture. Certain stainless allovs may have their corrosion resistance greatly reduced by carbide precipication in the heat-affected zone and the area along grain boundaries of ~:he HAZ is most liable .o m,icrocracking. r~;hi'e these effects can ~e controlled to some degree, com?romise of microstructure is 11;~1104 usually involved. Furthermore, these defects can ~e difficult to detect by routine nondestructive testing and may escape detection entirely.
Hot isostatic pressing (HIP) has been applied to the healing of void defects in castings. Improvements in fatigue properties of aluminum alloy castings have been achieved through HIP and are described in U.S. Patent 3,496,624, February 24, 1970. The application of hot isostatic pressing to superalloy (Rene' 80) (Rene' is a trademark of General Rlectric Company) castings has shown that the elimination of defects in cast superalloys generally increased stress-rupture, elongation and low-cycle fatigue life. HIP is also used in the preparation of shapes from various metal powders and for improving the ductility of shapes which have been made of metal powders.
SUMMARY OF THR INVENTION
It has been found that, by subjecting weldments of nickel-based alloys and stainless steel alloys to a sustained sub-stantially isostatic pressure at an elevated temperature, it is possible to reduce microcracks in the heat affected zone, eliminate or reduce the segregation and nonuniformities in the microstructure by enhanced diffusion, dissolve laves and other undesirable phases, reduce the amount and size of inclusions and improve tensile properties and impact strength of the weldments. By the process of this invention for improving the properties of weldments of nickel-based alloys and stainless steel alloys, the weldments are heated to a temperature suf-ficient to diffuse the material of the alloy and below the liquidus temperature of the alloy and simultaneously subjected to a substantially isostatic pressure sufficient to cause some plastic deformation of the material, the pressure and tempera-ture being maintained for a period of time sufficient to improve , :~

11~1104 the tensile properties and impact strength of the alloy o~ the weldment and render the material homo(3eneous.
It is, therefore, one object of the invention to provide a method for homogenizing the structure of the weldments of nickel-based and stainless steel alloys.
It is the other object of the invention to provide a method for improving the quality of weldments of nickel-based and stainless steel alloys by improving the tensile ductility and impact strength of the weldment.
DESCRIPTION OF THE PREFERRED EMBODIMENT
These and other objects of the invention for improving the quality of metal weldments of nickel-based and stainless steel weldments may be met by heating the weldment to about 1200 to 1480 K (1700 to 2200 F) in an inert or oxygen-free atmosphere while simultaneously subjecting the weldment to a substantially isostatic pressure of from about 34.5 to about 344.7 MPa (5000 to 50,000 psi) and maintaining the temperature and pressure for up to 8 hours whereby the weld and the heat-affected zone of the base metal have improved structural and mechanical properties.
The method of this invention is suitable for the improve-ment of weldments of any alloys, castings of which are known to be improved by hot isostatic processing. This includes, but is not limited to, the nickel and cobalt-based superalloys such as Inconel 718 and 625, (Inconel is a trademark of The Inter-national Nickel Co.) Rene' 41, Haynes 188 and 25, (E~aynes is a trademark of Cabot Corporation) and the austenitic (304, 316, 321, 347) or precipitation hardened stainless steels.
The temperature to which the weldment must be heated will depend upon the alloy being treated, bu-t generally must be high enough so that there will be diffusion of the constituents of the alloy while remaining below the liquidus or melting 11~1104 temperature of the alloy. For example, for Inconel 718, the temperature may range from about 1230K (1750F) to 1480K
(2200F) while for austenitic stainless steels, the range is from about 1200K (1700F) to 1420K (2100F). Temperatures of from 920 to 1145K (1200 to 1600F) are to be avoided to prevent sensitization of the Type 304 stainless steel.
The substantially isostatic pressure to which the weld-ments are to be subjected is dependent upon the temperature to which the weldment is heated, and must be slightly higher than the compressive yield strength of the material at the tempera-ture to which the weldment is heated. The pressure-temperature combination must be sufficient to result in plastic deformation of the weldment either by compressive yielding or by creep.
Generally pressures from about 34.5 to 344.7 MPa (5000 to 50,000 psi) will be adequate in the temperature range of 1200 to 1480K (1700 to 2200 F).
The period of time required is dependent upon the tempera-ture and pressure to which the weldments are subjected.
Generally the time must be sufficient to permit diffusion of the materials of the alloy to make the material homogeneous and to improve the tensile properties and impact strength of the alloy. Generally times of up to 8 hours have been found ade-quate.
The heating and isostatic pressing of the weldments may be applied by various means such as mechanical pressure application as by dies and fluid pressure application, as by an inert gas or a liquid, to prevent oxidation from taking place. Of these, the fluid isostatic pressure is the easiest to perform, by placing the weldment in a chamber which is pressurized to a level corresponding to the isostacic pressure desired while the weldment is maintained at the desired 11~110~

temperature. Autoclaves are generally able to provide suitable temperatures and pressures to achieve the desired results.
For those alloys which require heat treatment, such as the precipitation hardened alloys, it will be necessary to apply the second heat treatment procedure to the weldments after they have been hot isostatically pressed in order to ensure that the desired properties are retained by the alloys.
EXAMPLE I
To study the feasibility of hot isostatic processing of weldments, eight bars about 15.2 cm x 1.52 cm x 2.54 cm (6"
x 0.6" x 1.0") were sectioned from welded Inconel 718 plate.
The plate was welded with center-line U-grooved filled welds using Inconel 718 wire by gas tungston arc (GTA) welding. The bars were pressed at 103.4 MPa (15,000 psi) at 1365K (2000 F) for 1 hour. They were then allowed to cool at 85 K (150 F) per hour to 990K (1325 F). The plates were aged at 990K (1325 F) for 4 hours and then slow cooled at 55K (100 F) per hour to 895K
(1150 F) where they were held for 16 hours before cooling to room temperature.
The bars were then finish machined to tensile test speci-mens. Four specimens each were tested at room temperature (RT) and at 810K (1000 F). The results of the tensile test are shown in Table I.
All the specimens failed in a ductile manner in about the same location in the base metal, well outside the heat affected zone, showing tha~ the weld was stronger than the base metal.
Thus the data in Table I refer to the Inconel 718 base metal and not to its weldment. The scatter in tensile properties was minimal.

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EXAMPLE I I
In a manner similar to Example I, twelve additional speci-mens of Inconel 718 were provided with longituainal weldments.
The specimens were solution heat treated and hot isostatically processed at 1227K (1750 F) for one hour at 103.5 2~Pa (15 ksi) in an argon environment. The specimens were aged at 990K
(1325 F) for 8 hours, furnace cooled to 895K (1150 F), held at 895K (1150 F) for a total of 18 hours and air cooled to room temperature. The results of tensile tests at room temperature, 810K (1000 F), 920K (1200 F) and 1035K (1400 F), are given in Table II.
The longituainal welds èxhibited increasing reduction of area and elongation with increasing temperature from room tem-perature to 1035K (1400 F) and to 920K (1200 F), respectively.
A high elongation value of 11.8% at 1035K (1400 F), the duc-tility minimum temperature, and the reduced scatter in tensile data are definite advantages over unprocessed welds. -~1~1104 I I i I j I

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A number of longitudinal gas tungsten arc (GTA) weldments were made in Type 3G4 stainless steel plates 2.54 cm (1.0 in) thick using Type 308 stainless steel filler wire of 0.24 cm (0.094 in) diameter. mhe welded plates were then hot iso-statically pressed at either 1365K (2000 F) for 1 hour or at 1310K (1900 F) for 3 hours at 103.5,~lPa (15 ksi). Tensile test strength specimens per Federal Specification 151R2 with a gage length of 3.56 cm (1.4 in) and Charpy V-notch specimens were machined from welded plates before and after treatment.
The results of tensile tests at room temperature and at 590K
(600 F) are qiven in Table III. In Table IV are given the results of Charpy V-notch tests on Type 304/308 stainless steel welds before and after hot isostatic processing.
Thus it was shown that the hot isostatic pressing of Type 304/308 longitudinal GTA welds resulted in considerable increase in ductility and impact strength and reduction in scatter of the tensile properties.
Metallographic specimens were prepared of the treated Type 304 stainless steel weldments and compared with untreated Type 304 weldments. The comparison showed that void type defects such as cold laps were healed due to the treatment.
It was also shown that the processing homogenized the weld structure and converted the dendritic structure in the fusion zone to one with equiaxed austenitic grains. However, some grain growth in the heat affected zone and base metal occurred.

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In Table IV are given the results of Charpy V-r.otch tests on Type 304 stainless steel welds before and after treatment.
TABLE rv Notch in Impact Strength Spec. Fusion No. Condition Zone H.A.Z. Joules Ft. Lbs.
115 As welded X 206.0 152 116 As welded X 215.4 159 117 As welded X 207.3 153 118 As welded X 201.9 149 122 As welded X~135.3,~100 123 As welded X218.2 161 124 As welded X227.6 168 125 As welded X220.9 163 132 W-HIP-2000 X ~325.2 >240 138 W-HIP-2000 X'~325.2,~240 139 W-HIP-2000 X>325.2~240 143 W-HIP-2000 X~325.2~240 151 W-HIP-l900 X ~325.2 ~240 152 W-HIP-lgO0 ~. >325.2 ,240 160 W-HIP-l900 X~325.2,240 - Greater than the indicated value and means that there was no specimen failure - only bending of specimens.
Hot isostatic processing of Type 304 stainless steel longi-tudinal GTA weldments (with Type 308 stainless steel filler wire) resulted in:
Healing of cold lap or void type defects Homogenization of structure and grain growth Conversion of dendritic structure in fusion zone to one with equiaxed grains 30 to 50% increase in elongation with a corresponding 50 and 60~ decrease in yield strength at RT and at 590R (600 F) respectively Increase in RT Charpy V-notch impact strength from 210 joules (155 ft. lbs.) to more than 325 joules (240 ft. lbs.) The tensile properties of the HIPed longitudinal welds were equivalent to those of the base metal.

11~1104 - - - EXAMPLE IV
Two defective welded pipes, Bl and B2, with about 10 to 15~ porosity, were prepared from four Type 304 stainless steel pipe pieces. The dimensions of each pipe piece were: 14.0 cm (5.5 in) O.D., 1.9 cm (0.75 in) wall thickness and 10.16 cm (4 in) long. One end of each pipe piece was bevelled to 37.5 degrees, with a maximum land of 0.14 cm (0.055 in). The bevelled pipe pieces were cleaned and GTA welded (using Type 308 filler wire) so as to have 10 to 15% porosity. Porosity was obtained by using moisture, oily surfaces or inadequate purging with argon. Any surface porosity, as detected by liquid penetrant examination, was GTA weld sealed. The welded pipe Bl was sectioned into two halves BlA and BlB. The pipe half BlB was hot isostatically processed at 1365K (2000 F) for 1 hour at 103.5 MPa (15 ksi). Tensile test specimens were prepared from the as welded pipe half BlA and the welded and treated pipe half BlB. Room temperature tensile test results are given in Table V. The ferrite number (FN) calculated from delta-ferrite contents (measured with a Twin City Ferritmesser) are also included in Table V.
The processing resulted in healing of porosity, homogeni-zation of structure, conversion of dendrite into equiaxed austenitic structure and reduction of average F~l from 8.7 to 0.9. Decreasing ferrite content generally results in increasing ductility and creep life.
Treatment of defective pipe welds increased room tempera-ture elongation and ultimate strength by 140% and 10%, respec-tively, but decreased yield strength by about 39~ (Table V).
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4 MPa (33.5 + 0.6 ksi) which is greater than the minimum code - 0.7 value of 207 MPa (30 ksi) for mill-annealed Type 304 stainless ~131~04 steel. Additionally, the tensile properties of the welds showed a considerable reduction in scatter which means greater reli-ability. These benefits of hot isostatic pressing for defective welds show the potential for weld repair (healing void type defects) in the solid state.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for improving the physical properties of weldments of nickel-based alloys and stainless steel alloys comprising:
heating the weldments to a temperature high enough to diffuse the materials in the alloy and below the liquidus tem-perature;
simultaneously applying sufficient essentially iso-static pressure to the weldment to result in plastic deformation of the material; and maintaining the temperature and pressure for a period of time sufficient to reduce any porosity and render the alloy more homogeneous as characterized by an increase in tensile ductility and impact strength over weldments not subjected to the heat and pressure application.

2. The method of claim 1 wherein the weldment is heated to a temperature from 1200 to 1480 K and the essentially iso-static pressure is from 34.5 to 344.7 MPa.

3. The method of claim 2 wherein the temperature and pressure are maintained for a period of about 1 to 8 hours.

4. The method of claim 3 wherein the essentially iso-static pressure is applied by the action of a fluid.

5. An improved weldment of nickel-based alloy or stainless steel alloy having a homogeneous structure essen-tially free of microcracks prepared by heating the weldment to a temperature high enough to diffuse the materials in the alloy and below the liquidus temperature of the alloy while simultaneously applying sufficient essentially isostatic pressure to the weldment to result in plastic deformation of the material for a period of time sufficient to reduce porosity and render the material more homogeneous charac-terized by an increase in tensile ductility and impact strength over weldments not subjected to the heat and pressure application.

6. The improved weldment of claim 5 wherein the weld-ment has been heated to 1200 to 1480 K and subjected to a pressure of from 34.5 to 344.7 MPa.