JPS63309392A - Filler material for tig welding for austenitic heat resistant alloy - Google Patents

Abstract

PURPOSE:To improve creep characteristic, oxidizing resistance, ductility and crack resistance for weld part at high temp. by specifying composition of filler material at the time of welding to heat resistant alloy for using to boiler, etc., of thermal power station. CONSTITUTION:The composition of the filler material is made to 0.02-0.15wt.% C, 0.1-3.5% Si, 0.3-1.5% Mn, 18-35% Cr, 16-50% Ni, 0.5-3.0% Mo, 0.01-0.3% V, 0.01-0.5% Ti, 0.01-0.5% Nb, 0.003-0.01% B, <=0.04% P, <=0.005% S, 0.02-0.4% N and the balance Fe with inevitable impurities. In austenitic alloy, V and N are coexisted and Mo, Ti, Nb, V and B are added as complexing and by executing entering into solid solution, precipitation and stregthening of grain boundary of these elements to the weld metal, the creep rupture strength is improved. Therefore, the reliability of the weld joint can be remarkably improved.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a filler material for welding austenitic heat-resistant alloys used in boilers for thermal power generation and nuclear power generation, and more specifically relates to a filler metal for welding heat-resistant austenitic alloys used in boilers for thermal power generation and nuclear power generation. The present invention relates to a filler metal for TIG welding that provides a weld metal with excellent properties, oxidation resistance, toughness, and crack resistance.

[Prior Art] In recent years, as thermal power plants have become larger, boilers have tended to be used at higher temperatures and higher pressures. The increase in plant efficiency obtained by increasing the temperature and pressure is, for example, increasing the steam temperature from the current 538°C to 650°C and increasing the pressure to 3500 ps.
It is said that when increasing from ig to 5000 psiH, it is about 7%.

Development of heat-resistant alloys for boilers that can be used under such steam conditions is underway.

If the steam temperature is 650℃, the boiler metal temperature is 72℃.
Although the temperature will be around 0℃, it is a thermal material that can withstand this usage temperature! t Conventional 5tlS347, 5US316.5
US310 austenitic steel is insufficient, and a material with even higher strength is required. Furthermore, conventional austenitic stainless steels such as 5TLS347 and 5LIS316 have been developed with a focus on corrosion resistance at room temperature. From this perspective, considerable progress has been made in research and development to develop heat-resistant alloys that have the performance necessary for boiler materials in terms of high-temperature strength, high-temperature corrosion resistance, weldability, etc., with the addition of component studies to withstand high-temperature use. It's coming.

Under these circumstances, it has become necessary to develop welding filler metals with excellent oxidation resistance, high-temperature strength properties including creep properties, toughness, and cracking resistance. , a filler material for welding a Ni-based heat-resistant alloy has been proposed in which a weld metal free of defects in creep characteristics can be obtained by containing a trace amount of B in the Ni-based heat-resistant alloy. Furthermore, Japanese Patent Publication No. 61-25472 discloses a technology for a TIG welding filler metal of a nickel-based heat-resistant material containing specific percentages of C, Ti, Zr, Cr, Mo, etc., and the balance being Ni. It is not intended for the aforementioned heat-resistant alloys for boilers (due to cost issues due to the Ni base, and we have not yet provided a filler metal for TIG welding for austenitic heat-resistant alloys that has excellent creep strength and cracking resistance). .

For these reasons, it is desired to develop filler metals for TIG welding that have high creep strength.

[Problems to be Solved by the Invention] In view of the above circumstances, the present invention provides a method for welding austenitic heat-resistant alloys by using a T having a high creep strength.
The present invention provides a filler metal for IG welding.

[Means for Solving the Problems] The gist of the present invention is: ■C in weight percent:
0.02-0.15%, Si: 0.1-3.5%.

Mn: 0.3-1.5%, Cr: 18-30%, N
i: 16-50%, Mo: 0.5-3.0%,
V: 0.01-0.3%.

Ti: 0.01-0.5%, Nb: 0.01-0.
5%, B: 0.003-0.01%, P: 0.
0.04% or less, S: 0.005% or less.

An austenitic heat-resistant alloy characterized by containing 0.02 to 0.4% of N, and (1) further containing 0.1 to 6.0% of W, with the balance consisting of iron and inevitable impurities. It is a filler metal for TIG welding.

Effective means to increase the creep strength of weld metal are (a) strengthening by creating a solid solution, (b) strengthening by creating fine precipitates, and (c) increasing the strength of grain boundaries. As a result of intensive research, the present inventors have found that by coexisting austenitic alloys V and N, Mo,
W, Ti, Nb.

It has been found that the combined addition of V and B is effective. Basically, Mo and W are dissolved in the weld metal, and TI, Nb,
It is effective to finely precipitate carbides and nitrides with V and increase grain boundary strength with B to improve creep rupture strength.

Increasing Cr content is effective in improving high-temperature corrosion resistance, but this causes problems such as a decrease in creep strength due to lowering the stability of austenite, or a decrease in toughness and strength due to the formation of zero phase.

In order to prevent this, it is necessary to contain Ni in an amount commensurate with the amount of Cr. The present inventors have also discovered that Si is extremely effective in such high-temperature corrosion resistance.

The present invention has been made based on this knowledge, and the present invention will be described in detail below along with its effects.

[Function] The greatest feature of the present invention is that V and N coexist in the austenite alloy, and by adding Mo, W, Ti, Nb, V, and B in combination, solid solution, precipitation, and precipitation of these elements are prevented in the weld metal. The aim is to increase creep rupture strength by strengthening grain boundaries.

Next, the reasons for limiting each component will be described.

C, 0.02-0.15% Since the shape and distribution of carbides in C has a great effect on creep rupture strength and elongation at break, Cr and Mo.

It is necessary to add Ti, B, and Nb in the minimum amount necessary to form a carbide that is effective for creep properties. On the other hand, in order to prevent welding hot cracking, it is necessary to reduce the amount of C as much as possible. From the above viewpoint, the lower limit of C is set to 0.02%.
, the upper limit was set at 0.15%.

Si: 0.1-3.5% Si is based on the following experiment. Figure 1 shows 0.
05 Zero C, 1, OX Mn, 20X Cr, 259
gNi, 0.596Mo, 0.01e4V, 0.
05 zero Ti, 0.2X Nb, 0.005X B.

0.0296P, 0.002 zero S, 0.21N filler material consisting of high N addition balance iron and unavoidable impurities (inside the figure), 0.1X G, 1. Ok Mn, 21
X Cr, 25% Ni.

0.5 zero Mo, 0.01 zero V, 0.1 zero Ti, 0
．． 2 zero Nb, 0.004 zero B, 0.02 zero P, 0
．． Using a filler metal (marked 0 in the figure) consisting of 0.05 S, 0.05 zero N of normal N-containing balance iron, and unavoidable impurities, the amount of Sl was varied for two levels of different amounts of N, and this After solution treatment at 1150° C. for 30 minutes, the structure was examined and the relationship between crystal grain size and Si content is shown. As can be seen from the figure, when the Si content is increased, the crystal grains become larger and the crystal grain size number according to JIS regulations decreases.

This tendency is more pronounced than when the N content is 0.05% (○ mark in the figure).
．． The number of 2N (inside the mouth in the figure) is smaller. It can also be seen that the crystal size is smaller in the high N material and that N suppresses the coarsening of crystal grains due to increased Si content. Furthermore, as a result of examining the structures of a large number of materials, it was found that in materials of this component system, those with a crystal grain size number of 5 or less have a strong tendency to form coarse crystal grains locally and become mixed grains. Furthermore, as a result of the creep rupture test, it was found that the creep rupture strength of such mixed grains and non-uniform sizes decreased.

From FIG. 1, in order to avoid a decrease in creep rupture strength due to mixed grains, for example, in the case of a filler metal with an N content of 0.2, the Si content should be increased to 1. It can be seen that it is necessary to suppress the value to below i.

From the above observation results, it was found that the addition of Sl promotes coarse grain formation and mixed grain formation, and that this tendency is suppressed by increasing Nfi. In order to suppress this coarsening and mixing of grains, S
The ratio of i and the amount of N is % by weight, and the specific relational expression N* and 0.
01+ 0. Must be within the range that satisfies I5i96,
Zarani N is 0.4 to increase high temperature creep strength! The upper limit of Si was set at 3.596 using the above formula based on experimental results showing that it is effective up to ¥. In addition, Si is used as a deoxidizing agent, and in order to obtain fluidity of the molten metal, it is necessary to keep the content in the filler metal at 0.196 or more.
The lower limit of is set to 0.196.

Mn: 0.3 to 1.5% Mn sufficiently deoxidizes and fixes the S component, which is necessary to obtain a sound weld metal and is contained as an impurity, and improves weldability. The above is necessary. However, if the amount added is too large, the oxidation resistance will be impaired, so the upper limit should be set at 1.5.
It was set at 96.

Cr: 1B to 30% Cr improves high-temperature creep strength, high-temperature oxidation resistance, etc., so it is an essential element for filler metals for heat-resistant alloys. Since high-temperature oxidation resistance equivalent to or higher than 5US347 is required, the lower limit of Cri is set to 10
And so. However, if the amount of Cr is large, zero embrittlement is likely to occur due to long-term heating. 25 for alloy steel containing 50% Ni
Cr-20Ni austenite stainless steel! 1isU
In order to ensure zero embrittlement characteristics of s310 or higher, the upper limit of the Cr content was set to 30 zero.

Ni: 16-50% When Ni is added to steel at 10% or more, it changes the steel with a body-centered cubic structure into a steel with a face-centered cubic structure, so it is an essential element to ensure stable high-temperature strength. In order to suppress σ embrittlement that occurs in high Cr heat-resistant alloys that are used at high temperatures for long periods of time, it is necessary to add 167 or more. However, when the amount of Ni is large and the austenite becomes stable, work hardening tends to occur and hot workability deteriorates, which is not preferable from the viewpoint of wire drawing production. Also, in terms of cost, the larger the amount of Ni, the more expensive it becomes. For the above reasons, the upper limit of the Ni amount was set to 5096.

Mo: 0.5 to 3.0% Mo is an element necessary to increase creep rupture strength through solid solution hardening and precipitation hardening, but if it is less than 0.5, it will have little effect, so the lower limit of the amount added should be set. It was set to 0.5 zero. However, Mo has a strong tendency to segregate, and under high temperature and high pressure, it may promote sigma in the segregated parts, making it easy to cause local cracks and corrosion. Therefore, the upper limit of the amount added was set at 3.0 zero.

V: 0.01 to 0.3 forms stable precipitates during high-temperature creep and increases creep rupture strength. Figure 2 is o, os96c.

0.5t Si, 1. Fd Mn, 2H(:r,
10% Nl, 0.03 zero P.

0.057 Mo, 0.002 zero Ti, 0.0005
Zero B, 0.004 zero S.

Regarding weld metals containing 0.0211n N and varying the amount of V with filler metals consisting of iron and unavoidable impurities,
The graph shows the relationship between creep rupture time and V amount when creep rupture occurs under creep conditions of 550°C and 31 kgf/mm2. As can be seen from the figure, the creep rupture time becomes longer when V is added, but 0. No increase in creep rupture time was observed even when added in excess of tuna. This is V
This is because the precipitates containing N are thermally stable and contribute to strengthening the creep rupture strength over a long period of time.
In the case of , if the amount of V is added in excess of 0.3, the precipitates tend to become coarser, which not only reduces the effect of increasing the creep rupture strength, but also causes the creep rupture strength to deteriorate due to the coarser precipitates. There are cases where In addition, when V is less than 0.01, precipitates containing ■ are difficult to form, and the effect of increasing creep rupture strength is small.
The lower limit of quantity is 0.05 and the upper limit is 0. I made it eight.

Ti: 0.01 to 0.5% Nb: 0.01 to 0.5% Ti and Nb are carbonitride forming elements and have been conventionally recognized to be effective in improving creep rupture characteristics. Ti,
If the amount of Nb is less than 0.017, it has little effect on high temperature creep rupture strength. If it exceeds 0.564, coarsening of carbon, nitrides, etc. tends to occur, reducing creep rupture strength. For the above reasons, the lower limits of Ti and Nb were set to O, respectively, and the upper limit of Ol was set to O8.

B: 0.003-0. O1% B is necessary to increase the creep strength in an amount of 0.003 or more, but if the amount added is too large, weldability and ductility deteriorate, so the upper limit of the amount added was set to 0, ot96.

P: 0.04% or less If P is added in a large amount, it promotes precipitation during creep and promotes creep embrittlement, so the upper limit was set at 0.04 zero.

S: 0.005% or less S also segregates at grain boundaries and promotes embrittlement of grain boundaries during creep, so the upper limit was set at 0.005% F.

N: 0.02-0.4% N is known to increase the high-temperature creep strength of high-C, high-Ni austenitic alloys.
96C, 0.5 Zero Si, 1.0! kMn, 0.0
294P.

0.002 Zero S, 25 Ni, 20 Zero Cr, 1.5
Zero Mo, 0.2! kNb.

Filler metals consisting of 0.196Ti, 0.0057B balance iron and unavoidable impurities with N content varied from 0.02k to 0.40 were heated at 750℃ and 12kgf/m.
m” creep rupture test was conducted, and the creep rupture time and N
This shows the relationship with quantity. As the amount of N increases, the creep rupture strength gradually becomes stronger, but when the amount of N increases beyond 0.396, the tendency for creep rupture strength to increase decreases,
Even if it is added in excess of 0.4, no effect of increasing creep rupture strength can be expected, and creep rupture elongation also deteriorates.

Further, if N is less than 0.02, no effect of increasing creep rupture strength can be expected. For the above reasons, the upper limit of N is set to 0.
．． 4 zero, and the lower limit was set to 0.02 zero.

W: 0.1 to 6.0% Furthermore, in the present invention, stomach can be added especially for the purpose of improving creep rupture properties at high temperatures and long periods of time. W exhibits excellent high-temperature properties when added in combination with λio, and has no effect if the amount is less than O.
If it exceeds 0 zero, it will adversely affect the oxidation resistance, so the upper limit of W was set at 6.09 g, and the lower limit was set at 0.1 zero.

Next, the effects of the filler metal of the present invention will be described in more detail with reference to Examples.

[Example] A melted material with the same composition as the filler material was rolled into a plate thickness of 205 m, and the fourth
Bevel as shown in the figure (thickness T = 20 mm.

Bevel angle θ depth 20″″, root gap L x 14m
m), wire diameter 1.6φ with the composition shown in Table 1.
Filler metal in mm, welding electric current f! 150~220A, voltage 8~
T under welding conditions of 11v, welding speed 6-12cm/win
IG welding was performed.

In Fig. 4, 1 indicates the material to be welded, and 2 indicates the backing material.

Table 1 shows the creep rupture time and elongation at break of all weld metals at 750°C and a stress of 12 kgf/mm''.

Among the filler metals shown in Table 1, sample numbers 1 to 6 are comparative materials, 1 is 5US347.2 is 5tlS304 equivalent material,
3 has 25Ni-20Cr as the basic component W,
V, TI, Nb, and B are not added. Sample numbers 4 and 5.8 are both Ni-Cr based austenitic filler metals to which V, Ti, Nb, or B is not added. Sample numbers 7, 8, 9, 10, and 11 are filler metals of the present invention that fall under claim (1).

Sample number 7.8 has N of 0.05 zero and V of 0.0.
1.0.2! It was added to. ■Compared to sample No. 6 without additives, the strength is increased and the creep rupture elongation is also increased. Sample number 9. lO is V 0.2396 and N 0.1
21.0.231.If 0.4% is added and v is present, there is little decrease in elongation even if N is added to increase the strength.

In addition, sample number 11 has Si reduced to 1.0, but by adding 0.496 g of N, the decrease in strength due to increase in SL is suppressed.

Sample number 12.13 falls under claim (2). Sample No. 12 has an increased amount of W to 3 zero, and the addition of W increases the creep strength. Sample number 13 has a high Ni content of 5096. Ni
By increasing , the decrease in elongation at break is suppressed.

[Effects of the Invention] The filler metal of the present invention has significantly increased creep strength at high temperatures. As shown in Table 1, it is clear that weld filler metal compositions that meet the requirements of the present invention have superior high-temperature creep properties compared to those that do not meet the requirements of the present invention (comparative examples). . By using the filler metal according to the present invention when TIG welding austenitic steel used in various power generation boilers, the reliability of welded joints can be significantly improved.

[Brief explanation of the drawing]

Figure 1 shows the relationship between Si content and grain size, Figure 2 shows the effect of V on creep rupture strength, Figure 3 shows the effect of N on creep rupture strength, and Figure 3 shows the effect of N on creep rupture strength. FIG. 4 is a sectional view showing the groove shape of the welded portion used in the example. 1... Material to be welded 2... Backing material Fig. 1 81 content (%)

Claims (2)

[Claims] (1) By weight (%): C: 0.02-0.15% Si: 0.1-3.5% Mn: 0.3-1.5% Cr: 18-30% Ni: 16-50% Mo: 0.5-3.0% V: 0.01-0.3% Ti: 0.01-0.5% Nb: 0.01-0.5% B: 0.003-0.01% P: 0.04% or less S: 0.005% or less N: 0.02-0.4% A filler for TIG welding for austenitic heat-resistant alloys, characterized in that the balance consists of iron and inevitable impurities. Material. (2) By weight (%): C: 0.02-0.15% Si: 0.1-3.5% Mn: 0.3-1.5% Cr: 18-30% Ni: 16-50% Mo: 0.5-3.0% W: 0.1-6.0% V: 0.01-0.3% Ti: 0.01-0.5% Nb: 0.01-0.5% B: 0.003-0.01% P: 0.04% or less S: 0.005% or less N: 0.02-0.4%, with the balance consisting of iron and inevitable impurities. Filler metal for TIG welding for austenitic heat-resistant alloys.