DE3015638A1 - METHOD FOR IMPROVING THE PHYSICAL PROPERTIES OF THE HEAT-DAMAGED WELDING ZONE - Google Patents

Description

Nickel-based alloys and stainless steel alloys are high-strength corrosion-resistant materials, which are used in a wide temperature range. Because of their many desirable qualities, these Materials used extensively in the construction of nuclear reactors. Although both types of alloys with any arc welding process, such as gas-tungsten arc, can be easily welded, join These materials exhibit microcracks in the welds, a segregation of carbides and the formation of Slag or embrittling phases, as well as a reduction in ductility and of ductility-dependent phases Properties such as impact resistance and fracture toughness. These effects are most pronounced in the area of the base metal that emerges as a result of exposure to the heat of welding a metallurgical Has undergone change and is known as the heat damaged zone (HAZ). The metal in this zone was heated and cooled through a range of temperatures large enough to cause physical changes in the structure of the metal with consequent changes in the physical properties. In the case of steels, a hard zone is created depending on the extent to which they are concerned Steels are hardened. This is accompanied by a loss of ductility, so that the resistance against shock loads or fatigue is reduced. Brittleness can also occur as a result of the development a coarse-grained structure. Various stainless alloys can be heat-damaged by carbide precipitation Zone received a greatly reduced corrosion resistance and in the area along the grain boundaries of the HAZ microcracks often occur. While these effects can be controlled to some extent,

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There is often a compromise with regard to the microstructure. Moreover, such damage can be difficult can be discovered through non-destructive routine tests and can therefore go completely unnoticed.

Has been used to heal cavity defects in castings a hot isostatic pressing (HIP) has already been applied. As described in U.S. Patent 3,496,624, through HIP Improvements in the fatigue properties of aluminum alloy castings be achieved. The use of hot isostatic pressing in superalloy (Rene 80) castings has shown that the elimination of defects or defects in cast superalloys in general the breaking strength, elongation and service life for has increased slight alternating stress. HIP is also used in the formation of shapes from various metal powders and used to improve the ductility of molds made from metal powders.

It has been found "by treating the welds of nickel-based alloys and stainless steel alloys." with an extended, essentially isostatic pressure at elevated temperature is possible, microcracks in the heat-damaged zone to lessen the segregation and irregularities in the microstructure through increased diffusion to eliminate or reduce slag or other undesirable phases that dissolve To reduce the amount and size of inclusions, and the tensile properties and impact strength of the welds to improve. In the method according to the invention for improving the properties of the welds of alloys nickel-based and stainless steel alloys, the welds c-.i are heated to a temperature that sufficient to diffuse the material of the alloy

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or to distribute, but is below the liquidus temperature of the alloy, and at the same time a substantially exposed to isostatic pressure sufficient to cause a certain plastic deformation of the material cause; the pressure and temperature are maintained for a period of time sufficient to maintain the Tensile properties and impact strength of the alloy to improve the weld and to homogenize the material.

It is therefore an object of the invention to provide a method for homogenizing the structure of the welds of alloys nickel-based and stainless steel alloys.

It is another object of the invention to provide a method of improving the quality of welds of alloys to create nickel-based and stainless steel alloys, in which the tensile ductility and the impact resistance the weld is improved.

These objects are achieved according to the invention in that heat the welds to approximately 1,200 to 1,480 K (1,700 to 2,200 ° F) in an inert or oxygen-free atmosphere and at the same time subjected to a substantially isostatic pressure of about 34.5 to about 344.7 MPa (5000 to 50,000 psi) and kept at this temperature and pressure for up to eight hours, thereby creating the weld and improves the heat-damaged zone of the base metal in structural and mechanical properties will. The method according to the invention is suitable for improving welds of any alloys, castings of which are known to be improved by hot isostatic processing. this applies

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also without limitation, however, for the super log ieri based on nickel and cobalt, such as Tnconel 718 and 625, Rene 41, Haynes 188 and 25 and the austenitic (304, 316, 321, 347) or precipitation hardened stainless steels.

The temperature to which the weld must be heated depends on the alloy being treated, but it must generally high enough that diffusion of the constituents of the alloy occurs, but below that Liquidus or melting temperature of the alloy. For example, for Inconel 718, the temperature can be between 1230 K

(175O ° F), and 1480 K (22OO ° F), while for austenitic stainless steels which range between approximately 1200 K (1700 ° F) and 14 20 K (2100 ° F). Temperatures from 9 20 up to 1145 K (1200 ° to 1600 ° F) should be avoided in order to obtain a Avoid sensitizing the Type 304 stainless steel.

The substantially isostatic pressure to which the welds are exposed depends on the temperature to which the weld is heated; it must be slightly higher than the compression yield point of the material at the temperature to which the weld is heated. The pressure-temperature combination must be sufficient so that results in plastic deformation of the weld either by compression flow or by creep. In general Pressures range from about 34.5 to 344.7 MPa (5000 to 50,000 psi) in the temperature range 1200 to 1480 K (1700 to 22OO ° F).

The length of time required depends on the temperature and pressure to which the welds are exposed. Generally the time must be long enough for diffusion

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sion of the material of the alloy is possible to make the material homogeneous and the tensile properties and improve the impact resistance of the alloy. Times up to 8 hours are usually sufficient.

The welds can be heated and isostatically pressed in various ways, for example by exerting mechanical pressure by means of fittings and by applying pressure with a fluid, such as an inert one Gas or liquid to prevent oxidation. One of these possibilities is isostatic fluid pressure the easiest way to achieve this is to place the weld in a chamber that is under pressure, which corresponds to the desired isostatic pressure while maintaining the weld at the desired temperature will. Autoclaves are generally able to provide suitable temperatures and pressures in order to to achieve the desired results.

For those alloys that require heat treatment, such as the precipitation hardened alloys, it will be necessary to subject the welds to a second heat treatment after they are hot isostatic pressed to ensure that the desired properties of the alloys are maintained.

According to the invention, the tensile ductility and the Impact resistance of welds of nickel-based alloys and stainless steel alloys improved, so that they achieve the properties of the undamaged base metal. In addition, the welds are increased Subjected to isostatic pressure for a period of time sufficient to keep the material in the temperature Make welding more homogeneous.

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EXAMPLE I

To study the feasibility of hot isostatic Treating welds were eight rods approximately 15.2 cm by 1.52 cm by 2.54 cm (6 inches by 0.6 inches χ 1.0 in.) from a welded Inconel 718 plate. The plate was welded with a filled one Centerline U groove weld made using Inconel 718 wire gas tungsten arc odor welding (GTA) was produced. The bars were pressed at 103.4 MPa (150 Opsi) at 1365 K (2000 ° F) for 1 hour. then They were allowed to cool to 990 K (1325 F) at 85 K (150 ° F) per hour. The plates were left for 4 hours aged at 99ü K (1325 ° F) and then slowly cooled at 55 K (100 ° F) per hour to 89 5 K (1150 ° F) where they were Maintained for 16 hours before cooling to room temperature became.

The bars were then machined into tensile test specimens finely machined. There were four samples each at room temperature (RT) and at 810 K (1000 ° F) checked. The results of the tensile test are shown in Table I.

All samples failed in a ductile manner at about the same location on the base metal, well outside of the heat-damaged zone and thus showed that the weld was stronger than the base metal. The in Table I. The data given therefore relate to the Inconel 718 base metal and not on its weld. The scatter in the tensile properties was minimal.

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TABLE I.

sample
No. Temp.
K ( ⁰ F) End
strength (Ftu)
MPa (ksi) - 1146 (16fi, l) 0.2% stretch
limit (Fty)
MPa (ksi) (14 8.9) strain Flat.
reduction I. 37 R. T. 1367 (198.1) 1149 (166.5) 1027 (146.9) 20.2 31.3 38 Il 1357 (19C, 6) 1150 (166.7) 1014 (147.9) 20.4 32.0 39 π 1365 (197.8) 1143 (165.6) 1020 (145.7) 20.5 30.2 40 Il 1347 (195.2) 1147_f 4 (166, 2+. 6) 1005 11 (14 7, 8_f_l, 6) 20.4 30.7 ο Average 135 ^ + 10 (196.9 _ + _ 1 .5) 1016 + 20.4 _ +. L 31, 3- + O, 7 GD (128.0) cn 41 810Κ (1000 ° F) 383 (125.6) 14.7 ^ _{1 7} 083 42 It 867 (125.9) 14.6 31.8 KO 43 Il 869 (127.5) 14.1 32.0 44 Il 880 (126.8 ^ 1.2) 15.3 3 4 _; 0 Average 8 7 5 + 8 14.7 +, 6 ■: 3V.L ⁴ / ^G >

CD CO OO

EXAMPLE IT

In a similar way to Bei spie] - 1, twelve additional samples made of Inconel 718 were provided with longitudinal welding suns. The samples were solution heat treated and hot isostatically processed at 1227 K (1750 ° F) for 1 hour at 103.5 MPa (15 ksi) in an argon environment. The samples were aged at 990 K (1325 F) for 8 hours, then oven cooled to 895 K (1150 ° F), then held at 895 K (1150 ° F) for a total of 18 hours, and finally air cooled to room temperature. The results of tensile tests at room temperature, 810 K (1000 ° F), 920 K (1200 ° F) and 1035 K (1400 ⁰ F) are given in Table II. The longitudinal welds showed an increasing reduction in area and elongation with increasing temperature from room temperature to 1035 K (1400 ° F) and up to 920 K (1200 ° F). A high elongation value of 11.8% at 1035 K (1400 ° F), the minimum ductility temperature, and the reduced scatter in tensile strength data are definite advantages over untreated welds.

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TABLE II

Test Temp. K (Op)

Ultimate Strength (Ftu) MPa (ksi) Yield Strength (Fty)
0.2% MPa (ksi)

Strain %

Measurement data

middle

Measured data mean

Measured data mean

RT

1231 (178.4) 1256 1287 (186.5) (182.0) 1249 (181.0) (154.0). 1072
(157.1) (155 _; 3)
(154.9)

7 _; 8th

8.7

810K 1073 (155.5) 1067 906 (131.3) 880 (1000 ° F) 1069 (155.0) (154.7) 883 (128.0) (127 _; 6) 1060 (153.6) 852 ( 123.5)

13.0 16.9 14.9

920K 1005 (145.6) 1010 814 (117.9) 830 (1200 ° F) 998 (144.7) (146.3) 809 (117.2) (120 _; 2) 1025 (148.6) 866 ( 125.5)

16.8 15 _; 6 12.7

1035K 684 (99.1)

(1400 ° F) 685 (99.2)

664 (96.3)

678 644 (93.3)

(98.2) 634 (91.9)

600 (86.9) 626
(90 _; 7;

12.3

11.9

11.2

9.4

14.6

15 _; 0

11.8

TABLE II (continued)

CO OO * - in

O OO Sx) IM

test
Temp.
K ( ⁰ F) Area reduction middle RT Measurement data 15.2 12 _; 8th 15.1 810K 8.2 (1000 ° F) 15.4 21.8 28.9 920K
(1200 ° F) 21.1 23.7 17.8
34.3 103 5K 19.1 (1400 ° F) 38.5 4 0 _; 3 3C, 9th 43 _; 5

CD CO OO

EXAMPLE III

A number of longitudinal gas tungsten arc (GTA) welds were made in 1.0 inch thick sheets of Type 304 stainless steel using a Type 308 stainless steel filler wire having a diameter of 0.24 cm (0.094 inch) was used. The welded plates were then hot isostatically pressed, either at 1365 K (200ü ° F) for 1 hour or at 1310 K (1900 ⁰ F) for 3 hours, and at a pressure of 103.5 MPa (15 ksi). The welded panels were made up to 3.56 cm (1.4 inch) gauge length and Charpy V-notch samples for tensile strength testing in accordance with Federal Specification 151R2 before and after treatment. The results of the tensile tests at room temperature and at 590 K (600 ° F) are given in Table III. Table IV shows the results of Charpy V-notch tests on 304/308 stainless steel welds before and after hot isostatic machining.

It has thus been shown that the hot isostatic pressing of the GTA longitudinal welds with Type 304/308 results in a considerable increase in ductility and impact resistance and a reduction in the dispersion of the Tensile properties led.

Metallographic samples were made from the treated 304 stainless steel welds and compared to untreated Type 304 welds. The comparison showed that due to this treatment pores or. Void defects such as cold welds were healed.

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The treatment was also shown to homogenize the weld structure and the dendritic structure
transformed into a structure with equiaxed austenitic grain in the melting zone. However, some grain growth occurred in the heat damaged zone and
the base metal.

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TABLE III

Test Temp. K (° F (

State

Mean value 0.2% Fty MPa (ksi)

Average

Ftu MPa (ksi)% Mean% Mean
Stretch in areas - 3.56cm (1.4in.) Reduction

PT RT RT

590K (600 F) 590K (600 ° F) 590K (600 F)

W 420 + 14 (60 _; 8 + _2.0) 624 + 6 (90

W-HIP-2000 210 + 4 (30.4 + 0.6) 589 + 3 (85 _; 3 + _0 _; 5)

W-HIP-1900 199 + 6 (28.9 + 0.85) 582 + 4 (84 _; 4 + _0 _? 6)

3, 6 + _2, 1

69 _; 5 + _2.7

72.2 ± 0.8

W 338 + 16 (49 _; 0 + 2.3) 442 + 11 (64.0 + 1 ^ 6) 29.2 + 3.2

W-HIP 2000 120 + _2 + 5 410 (59.4 + 0.7) 43 (17 + 0 4 _3?); 7 + .0.9

W-HIP-1900 116 + 6 (16.8 ^ 0.9) 407 + _0.3 (59.0 + 0.05) 45 5 + 0 5

ff f J »* {

73 _; 8_ + 2 _; 3
72.8 + 3.7
77 + 2.0

68

_; 5i

5, 5 + _l j 5

W = as welded

W-HIP-2000 = welded and then hot isostatically processed at 1365 K (200O ⁰ F) - 1 hour - 103.5 MPa (15 ksi)

W-HIP-1900 = welded and then hot isostatically machined at 1310 K (1900 ° F) - 3 hours - 103.5 MPa (15 ksi)

cn co 00

In Table IV are the results of the Charpy V-notch test on welds with Type 304 stainless steel indicated before and after treatment.

TABLE IV

State score in Impact resistance Ft. Lbs. sample like welded Enamel 152 No. like welded Zone H.A.Z. Joules 159 115 like welded X 2O6, O 153 116 like welded X 215.4 149 117 like welded X 207.3 > 1OO 118 like welded X 201, 9 161 122 like welded X > 135.3 168 123 like welded X 218.2 163 1 24 W-HIP-2OOO X 227.6 > 240 125 W-HIP-2OOO X 220.9 > 24O 132 W-HIP-2OOO X > 325.2 > 24O 138 W-HIP-2000 X > 325.2 > 24O 1 39 W-HIP-1900 X > 325.2 > 24O 143 W-HIP-1900 X > 325.2 ;> 24O 1 51 W-HIP-1900 X > 325.2 > 24O 152 X > 325.2 160 X > 325.2

= greater than the specified value and means that the specimen did not break, only a bending of the Rehearse.

A hot isostatic treatment of GTA longitudinal welds with type 304 stainless steel and a type 308 stainless steel filler wire gave the following results:

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Healing of cold welds or pore defects Homogenization of the structure and grain growth Conversion of the dendritic structure in the melting zone into a structure with an equiaxed grain an increase of 30 to 50% in the elongation with a corresponding decrease of 50 and 60% of the yield point at room temperature or at 590 K. (600 ⁰ F) increase in the Charpy V-notch impact strength at room temperature of 210 joules (155 ft.lbs.) to more than 325 Joule (240 ft.lbs.).

The tensile properties of the longitudinal welds treated with HIP were the same as those of the base metal.

EXAMPLE IV

Two incorrectly welded pipes, B1 and B2, with about 10 to 15% porosity were made from four pipe sections Type 304 stainless steel. The dimensions of each piece of pipe were: 14.0 cm (5.5 inches) outside diameter, 1.9 cm (0.75 inch) wall thickness and 10.16 cm (4 inch) length. One end of each piece of pipe was at 37.5 beveled with a maximum elevation of 0.14 cm (0.055 inch). The beveled pipe pieces were cleaned and GTA welded (using a filler wire of type 308) so that they had a porosity of 10 to 15%. The porosity was determined by using Moisture, oily surfaces or insufficient purging with argon achieved. Any surface porosity, determined by a liquid penetration test was sealed with a GTA weld. The welded pipe B1 was in half, B1A and B1B divided. The pipe half B1B was hot isostatically processed, namely at 1365 K (2000 ° F) for 1 hour with

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103.5 MPa (15 ksi). There were tensile test samples from the welded pipe half B1A and the welded and treated pipe half B1B. The results of Tensile tests at room temperature are given in Table V. The ferrite number (FN) derived from the delta ferrite content (measured with a Twin City ferrite meter) are also given in Table V.

The machining led to a healing of the porosity, a homogenization of the structure, a transformation of Dendrite into an equiaxed austenitic structure and a reduction in the mean FN from 8.7 to 0.9. The decreasing ferrite content usually leads to increased ductility and creep life.

Treating the defective pipe welds increased room temperature elongation and breaking strength by 140% and 10%, respectively, but reduced the yield strength by about 39% (Table V). The yield strength at room temperature of treated welds was 231 +4 MPa (33.5 + 0.6 ksi), which is greater than the minimum code value of 207 - 0.7 MPa (30 ksi) for roll-annealed stainless Type 304 steel. In addition, the tensile properties of the welds showed a significant reduction scatter, i.e. greater reliability. These advantages of hot isostatic pressing for faulty Welds show the possibility of a weld repair (healing of porosity defects) in the solid state.

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TABLE V

sample pipe BIB State {Y!} Fty (0 , 2%) , 3 Ftu MPa ksi , 0 ,8th % Strain No. BlA BIB W. MPa ksi ,1 593 86 ,8th 2.7 in 3.56 cm 40 BlA BIB Iv 338 56.2 558 80 5 (1.4 in) 41 BlA BIB W. 373 54.0 514 74 ,1 36 42 BlA ElB W. 372 53.9 449 65 > ⁶ 20th 43. BlA BIB W. 373 54.1 535 77 , 6 14th 44 BlA BIB W. 371 53.8 604 87 , ⁷ 9 45 BlA the end W. 386 56.0 502 72 8 + 9 18th 46 the end , 5 values W. 386 56.0 537 + 88 77 -1 30th middle 7 values 379 + 9 54.9 + 1 -67 13th -8th -1 20 + 16 , 6 ,1 »5 -11 O ⁴⁷ TK, W-HIP-2000 , 7 566 82 > 2 ω ₄₈ [b] w-riip-2000 230 33.4 581 84 , ² CD 49 _rL1 W-HIP-2000 231 33 5 574 83 > ⁶ 49 _r , CD 50 ^fb] W-HIP-2000 235 34 1 58 4 84 > ⁶ 80 f ° ^] £ Jt 51 W-HIP-2000 239 34.6 571 82 , 2 47 52 W-HIP-2000 235 34.1 574 83 , 6 66 ^fc] O 53 W-HIP-2000 226 32.8 570 82 7 + 0 50 OO middle W-HIP-2000 230 33.3 570 + 4 82, -0 50 (YES 231 + 4 33.5 + 0 f 46 -5 t 48.4 + 1.6 "2.4

£ äj W = like welded

W-HIP-2000 = welded and then hot isostatically machined at 1356 K (2OOO ° F) - 1 hour - 103.5 MPa (15 ksi).

/ b} Data for these two samples Nos. 48 and 50 were not included in the

Calculation of the mean tensile values taken into account.

(c] Base metal failure. All other samples failed in the

Weld fusion zone.

cn

OJ OO

TABLE V (continued)

Ferrite number FN

Sample (delta ferrite content of

No_. Welding ferrite knives )

⁴ 0 ■ 9.2

41 9 2

42 7.6

43 10.0

44 8 8

45 7.6

46 8.4 funds from

7 values 8.7 + 1, "3

^{ω 47} γκι 0.5

ο ₄₈ [b] _{0 4}

° ⁴⁹ fbi ² ' ⁴

cn 50 l ^b J 1J4

^ 51 0.8

52 0.4

S ⁵³ °; ³⁵ IK) funds

5 values 0.9 + 1.5

-0.55

With the preceding discussion and examples it has been shown that treating welds a remarkable improvement in ductility and impact strength through hot isostatic pressing and brings about a reduction in the dispersion of tensile properties of the alloys. Even the healing of porosity defects and a homogenization of the weld structure were demonstrated.

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Claims (7)

Claims Process for improving physical properties the heat-damaged zone of welds of nickel-based alloys and stainless steel alloys, thereby characterized in that the welds are heated to a temperature high enough to to diffuse the materials in the alloy, which, however, is below the liquidus temperature, that at the same time a sufficient, essentially isostatic pressure is applied to the weld so that a plastic deformation of the material and that the temperature and pressure are maintained for a period of time sufficient to reduce the porosity and the heat-damaged zone of the weld to become more homogeneous to make, as is evident from an increase in tensile ductility and impact resistance compared to welds that Ö30045 / 0832 Tt: iEFON IEl IiX: fi-i? 2O3il pulW d not treated by heat and pressure shows. 2. The method according to claim 1, characterized in that the weld to a temperature is heated from 1,200 to 1,480 K and the essentially isostatic pressure between 34.5 to Is 344.7 MPa. 3. The method according to claim 2, characterized in that the temperature and the pressure for a period of about 1 to 8 hours. 4. The method according to claim 3, characterized in that the substantially isostatic Pressure is applied by the action of a fluid. 5. Improved welding of a nickel-based alloy or a stainless steel alloy, characterized by a homogeneous structure, the is essentially free of microcracks and is produced by heating the weld to a temperature which is high enough to allow the materials in the alloy to diffuse, but which is below the liquidus temperature of the alloy, with an isostatic pressure on the weld at the same time that is large enough to cause plastic deformation of the material, for a period of time sufficient to reduce the porosity and make the material more homogeneous to make, so that there is an increase in tensile ductility and impact strength compared to welds which have not been subjected to the heat and pressure treatment. 030045/0832 6. Weld according to claim 5, characterized in that the weld to 1200 to 1480 K and a pressure of 34.5 to 344. 7 MPa. 0300A5 / 0832