JPH0787990B2 - Submerged arc welding method for high strength Cr-Mo steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to a high-strength, high-strength pressure vessel used for
The present invention relates to a submerged arc welding method for Cr-Mo steel, and particularly to obtain a weld metal excellent in creep strength, toughness, and resistance to embrittlement during use, including hydrogen cracking resistance.

(Prior Art) In recent years, in the field of oil refining, there has been an increasing trend to further improve the conventional operating conditions to increase the efficiency, and to reduce the weight of the plant to reduce the construction cost. The strength tends to be further increased. Cr-Mo used in heavy oil crackers and desulfurizers
As for steel, an improved steel capable of withstanding more severe conditions than conventional steels, that is, not only high strength but also excellent creep strength and hydrogen attack resistance (for example, JP-A-61
-223163) has been developed.

By the way, when welding Cr-Mo steel plates used for such applications, the plate thickness of the steel plates may reach up to about 300 mm, and the narrow merged submerged arc welding method and gas shield arc welding method are mainly used. However, the present situation is that the welding material for welding the newly developed high-strength Cr—Mo steel having the same characteristics as the steel sheet has not yet been developed.

In terms of strength, the weld metal must have the same creep rupture strength as the base metal, and the tensile strength at room temperature should be the same level as the base metal. That is, it does not result in an extreme overmatched joint. On the other hand, in toughness, heat treatment after welding (PWHT)
Good toughness after and after toughening treatment (step cleaning treatment), and small difference between toughness value after step cleaning and toughness value as PWHT, so-called embrittlement amount Furthermore, the amount of toughness embrittlement after exposure for a long time in a high temperature and high pressure hydrogen environment is small.

Furthermore, in a desulfurization reactor, etc., the sulfur content in heavy oil is removed as H ₂ S under high temperature and high hydrogen partial pressure, so the inside of the container is high in H ₂ S.
Since it is in the environment, there is a problem of so-called hydrogen cracking in which H ₂ S promotes hydrogen penetration into the material and causes cracking at shutdown. Such hydrogen cracking causes a brittle fracture starting from this crack and may cause destructive damage to the container, which is a serious problem. As a method of preventing this hydrogen cracking, for example, NACE (National Association of
Corrosion Engineering) has proposed that the hardness of the material should be 22 or less in H _R C for low alloy steel (Hv 248 or less when converted to Vickers hardness). Although this value is mainly applied to the line pipe material, the inventors conducted a 4-point bending SSC test as described below.
-Mo steel welded joints were investigated and hydrogen crack resistance was investigated.
Have found that it can be adopted as a limit value for preventing hydrogen cracking.

However, in the current Cr-Mo steel welding materials, there is no reference to the hardness of the weld metal. For example, JP-A-62-25
As can be seen in Japanese Patent No. 9695, although wires that have been strengthened by adding V, Nb, B, etc. are disclosed, the hardness of the weld metal far exceeds Hv248 due to excessive increase in room temperature strength. It is thought that

Furthermore, hydrogen cracking is Hv248 even at one point in weld metal.
The maximum hardness of the weld metal must be Hv248 or less because it may occur when the value exceeds. In order to obtain such a weld metal, it goes without saying that the composition of the steel plate and welding wire to be used is limited in consideration of dilution by the steel plate, the temperature between welding passes and the heat input for welding are limited, Furthermore, it is important to limit the heat treatment conditions after welding.

(Problems to be Solved by the Invention) The present invention, when submerged arc welding Cr-Mo steel, while maintaining the creep strength of the weld metal equal to the base metal,
It is an object of the present invention to propose a welding construction method for obtaining a weld metal which has excellent toughness and embrittlement resistance during use as well as hydrogen cracking resistance.

(Means for Solving the Problems) The gist of the present invention is C: 0.09 to 0.18 wt% (hereinafter simply referred to as%), Si: 0.13% or less, Mn: 0.25 to 0.65%, Cr: 1.85 to 3.2.
5%, Mo: 0.85 to 1.15%, V: 0.23 to 0.37%, S: 0.015% or less, P: 0.020% or less, tensile strength: 60 to 77.5 kgf / mm ² , 0.2% proof stress ≥ When submerged arc welding of steel plate with strength of 42 kgf / mm ² , C: 0.07 ~ 0.15%, Si: 0.30% or less as filler wire, M
n: 0.50% to 1.00%, Cr: 2.25 to 3.25%, Mo: 0.85 to 1.15
%, V: 0.20 to 0.35%, Nb: 0.03% or less, Ti: 0.005 to 0.02
% Ni: 0.60% or less, N: 0.005% or more, and contains less than 0.01%, the balance being substantially with use of wire made of the composition of _{Fe, MgO-BaO-SiO 2} -CaF 2 based high base as flux Welding is carried out under the conditions of preheating and interpass temperature: 175-250 ℃, welding heat input: 20-50kJ / cm, and then at least 670 ℃ in the temperature range of 670 ℃ or more for the welded part. The maximum hardness of the weld metal is set to Hv248 or less in the Vickers hardness test by subjecting the weld metal to post-weld heat treatment satisfying TP of 20.20 to 20.50, which is represented by the following formula (1). The purpose is to obtain a weld metal that is excellent in crackability, has the same creep strength as the base metal, and has excellent toughness and embrittlement characteristics during use.

Note TP = (T + 273) × (20 + logt) × 10 ⁻³ (1) where T: heat treatment temperature after welding (° C.) t: heat treatment time after welding (h) The present invention will be described in detail below.

Now, the inventors have studied various wire component systems, flux compositions, and welding conditions in detail, and further investigated the effect of post-weld heat treatment conditions on the maximum hardness of the weld metal, and determined the tensile strength of these weld metals. , Toughness, amount of embrittlement during use, creep rupture strength and hydrogen crack resistance were investigated. The specific actions of these will be described below.

(Operation) First, in this invention, C: 0.09 to 0.18%, Si: 0.13% or less, M
n: 0.25 to 0.65%, Cr: 1.85 to 3.25%, Mo: 0.85 to 1.15%,
The target is Cr-Mo steel having a composition containing V: 0.23 to 0.37%, S: 0.015% or less, and P: 0.020% or less. The present invention is based on the conventional steels ASTM A387 Gr21, Gr.22 and A336 F21, F22.
It is intended for materials used in higher temperature and high hydrogen pressure environments that cannot be applied with the materials specified by
This is because a net material in the above specified range in which V is added to the composition of Cr, Mo and the like to remarkably improve creep strength and hydrogen erosion resistance is indispensable for constituting the present invention.

Next, the wire composition will be described.

C is a useful component effective for improving the strength, but if it is less than 0.07%, the fine carbides such as V and Nb effective for improving the cleave strength are not sufficient, and the creep strength is insufficient. However, if added excessively, the strength and hardness will remarkably increase, and it may cause high temperature cracking, so the upper limit was set to 0.15%.

Si is an element that affects temper embrittlement,
From the viewpoint of embrittlement, it should be limited to 0.30% or less.

Mn effectively contributes to the improvement of toughness and strength, but if it is less than 0.50%, its addition effect is poor, while 1.00%
If it exceeds the range, toughness deterioration, especially embrittlement during use, is likely to occur, so the range was limited to 0.50 to 1.00%.

Cr and Mo are elements added in terms of oxidation resistance and high temperature strength, and are basic components of the Cr-Mo steel targeted by the present invention. Therefore, even in the weld metal, Cr should be 2.25 to 3.25%, and Mo should be the same as that of the base metal.
In addition, in the range of 0.85 to 1.15%.

V is added to the base material as an indispensable element from the viewpoint of creep strength and hydrogen attack resistance in the Cr-Mo steel targeted by the present invention. Therefore, in the weld metal as well, from the viewpoint of creep strength and hydrogen attack resistance, it is desirable to add the same amount as the base metal,
Taking into consideration the dilution with the base metal, if the composition of the welding wire is 0.20% or more, the creep strength and the hydrogen attack resistance of the weld metal will be good. On the other hand, if V is added excessively, not only the tensile strength at room temperature becomes too high and the strength range of the base metal is exceeded, but also the maximum hardness of the weld metal becomes high, and further the toughness is impaired.
For welding wires, the upper limit was limited to 0.35%.

Nb has the effect of improving the creep strength even if added in a small amount, but if added in excess of 0.03%, not only the tensile strength at room temperature but also the hardness becomes too high, and further the toughness is impaired. Limited to 0.03% or less.

Ti, like Nb, effectively contributes to the improvement of creep strength with a small amount of addition, but in order to obtain that effect, it is at least 0.
It is necessary to contain 005%. However, 0.02%
If added in excess of 10%, the tensile strength and hardness at room temperature will be too high, and the toughness will be impaired, so the content was limited to 0.02% or less.

Ni is a useful element that is effective in improving toughness after SR, but if added in excess, it causes embrittlement in a high temperature and high pressure hydrogen environment, so it was limited to 0.60% or less.

N combines with V, Nb, etc. to form fine nitrides or carbonitrides. These have a remarkable effect on the improvement of creep strength, but in order to obtain such an effect, N is added to the welding wire.
It is necessary to add 0.005% or more. However
If it is added in an amount of 0.01% or more, the tensile strength at room temperature becomes too high, the hardness becomes too high, and the toughness is impaired. Therefore, the addition amount should be 0.005% or more and less than 0.01%.

Now described flux, bases in the present invention, it is necessary to use a MgO-BaO-SiO ₂ -CaF ₂ based highly basic firing type flux as the flux, as the basicity is indicated, for example, the following equation (2) In the formula of degree BL, BL:
It is preferably 2.3 to 4.5.

BL = (% MgO +% BaO +% CaO +% CaF ₂ ) / (% SiO ₂ +% A
l ₂ O ₃ +% TiO ₂ +% MnO +% ZrO ₂ ) (2) When BL is less than 2.3, the slag tends to become vitreous and the amount of oxygen in the weld metal increases, leading to deterioration of toughness. On the other hand, if it is more than 4.5, the melting point of the slag is increased and the slag peeling property is deteriorated, and there is a possibility that welding defects may occur especially in a narrow groove. In addition, since the firing type flux can easily add metal carbon salts and metal powders, which are gas generating components, it is possible to control the oxygen content in the weld metal to a low level, and also the hydrogen content can be reduced. .

The welding heat input greatly affects the toughness, strength and hardness of the weld metal. If it is less than 20 kJ / cm, defects are likely to occur in narrow groove welding, and also in terms of work efficiency.
20kJ / cm or more. On the other hand, if it exceeds 50 kJ / cm, the stacking amount per layer increases and the tempering effect depending on the reheating by the passes of the next and subsequent layers cannot be obtained, the hardness increases, and the toughness also deteriorates, so 50 kJ / cm Limited to cm or less.

The interpass temperature is 175 ° C or higher to prevent cold cracking and to prevent the hardness of the weld metal from rising excessively. On the other hand, if the temperature exceeds 250 ° C, the cooling rate of the weld metal increases and the toughness is impaired due to insufficient quenching, so the temperature was limited to 250 ° C or less.

In the post-welding heat treatment, when the characteristics of the weld metal were investigated at various treatment temperatures and times as described below, the maximum hardness of the weld metal exceeded Hv248 when the TP expressed by the following formula (1) was less than 20.20. Since it was found that the hydrogen cracking property deteriorates, TP ≧ 20.20. However, there are cases where post-weld heat treatment is performed several times during the welding process of the container, and the decrease in strength poses a problem, so the upper limit was set to TP20.50. In the above heat treatment after welding, the processing temperature was 670 ° C.
If it is less than TP, it takes a long time to satisfy the above range and is not suitable for practical work. Therefore, the post-weld heat treatment is performed at a temperature of 670 ° C or higher.

Note TP = (T + 273) × (20 + logt) × 10 ⁻³ (1) where T: post-weld heat treatment temperature (° C) t: post-weld heat treatment time (h) That is, TP is 20.20 to 670 ° C or higher in the temperature range. 20.5
By performing the post-weld heat treatment of 0 at least once, the maximum hardness of the weld metal can be set to Hv248 or less, and even if such post-weld heat treatment is performed several times, the strength decreases. There is no.

(Example) For steel plates having the chemical composition shown in Table 1 and the mechanical properties shown in Table 2, the wire shown in Table 3 and the flux shown in Table 4 were used, and the groove shape shown in FIG. Narrow groove submerged arc welding was performed under the welding conditions shown.

The heat treatment conditions after welding are as shown in Table 6.

The following tests were performed on the obtained weld metal as it was after the post-welding heat treatment shown in Table 6 and on the post-weld heat treatment that was further subjected to the step cooling treatment shown in FIG.

Tensile test is performed at room temperature and 480 ℃, and room temperature strength is 60 ~ 7
Those with 7.5 kgf / mm ² and those with a strength of 480 ° C of 52 kgf / mm ² or more were considered good.

Creep rupture strength is equivalent to 100,000 hours at 480 ℃ 5
The strength at 50 ° C for 800 hours was calculated by interpolation, and this value was 24 kgf / m.
Those with m ² or more were judged as good.

As for the toughness of PWHT, the minimum Charpy absorbed energy at −18 ° C. was 10 kgf · m or more. The toughness after step cooling was judged to be good when the following expression (2) was satisfied.

vTr _5.5 ＋3 ・ ΔvTr _5.5 ≦ 10 ℃… (2) where ΔvTr _5.5 = vTr ′ _5.5 −vTr _5.5 vTr _5.5 : The absorbed energy of the weld metal as it is PWHT is 5.5kgf
・ Temperature at which m reaches vTr ' _5.5 : The temperature at which the absorbed energy of the weld metal after the step cooling treatment becomes _5.5 kgf ・ m. Further, the evaluation of hydrogen corrosion resistance is at a temperature of 550 ° C and a hydrogen pressure of 500
A Charpy test piece taken from the weld metal was exposed in an autoclave maintained at kgf / mm ² for a certain period of time, and then a Charpy test was performed at 0 ° C to investigate the exposure time dependence of the absorbed energy, and the absorbed energy The time when the decrease started was defined as the latent period, and those with a latent period of 150 hours or more were judged to be good.

The hardness of the weld metal is 3rd in the Vickers test under a load of 10kg.
As shown in the figure, it was carried out at a 2 mm pitch in a grid pattern of 2 mm intervals, and the maximum hardness of them was the maximum hardness of the weld metal.

Furthermore, for the determination of hydrogen cracking resistance, a test piece taken from the vicinity of the cross section showing the highest hardness in the hardness test is used,
NACE liquid (25 ° C) with 80% stress of 0.2% proof stress
Of saturated H ₂ S + 0.5% CH ₃ COOH + 5% NaCl solution) for 1000 hours, and a 4-point bending SSC test was performed.

Example 1 Various wires and flakes F2 shown in Table 3 for steel A
Narrow groove 1 layer 1 pass multilayer welding was performed under the welding condition WC1.

The chemical composition of the obtained weld metal is summarized in Table 7.

Table 8 shows the results obtained by subjecting each of the above weld metals to the heat treatment of HT2 shown in Table 6 and then conducting various tests.

As is clear from Table 8, in No. 1 which is a conforming example, strength, toughness, aging resistance during use, hydrogen attack characteristics and hardness characteristics, and hydrogen cracking resistance characteristics are all good.
On the other hand, in Nos. 2 to 13, any one of the conditions is out of the proper range of the present invention, and therefore, as shown in the remarks of Table 8, not all the characteristics are necessarily satisfactory.

Example 2 Welding condition W using wire WR1 and flux F1 for steel A
Narrow groove 1 layer 1 pass multilayer welding was performed by C2.

Table 9 shows the chemical composition of the obtained weld metal.

Then, the weld metal was heat-treated under various conditions shown in Table 6, and various tests were carried out.
Shown in 10.

As is clear from Table 10, in No. 14 to 16 which are conforming examples, all of the results are good in strength, toughness, embrittlement resistance during use, hydrogen attack characteristics and hardness characteristics, and hydrogen cracking resistance characteristics. However, in Nos. 17 and 18 in which the heat treatment conditions are out of the range of the present invention, satisfactory characteristic values were not obtained.

Example 3 Steel B was subjected to narrow groove 1-layer 1-pass multilayer welding under various welding conditions shown in Table 5 using wire WR3 and flux F3.

Table 11 shows the chemical composition of the obtained weld metal.

Next, heat-treat HT1 for each of the above weld metals WM15-19
Table 12 shows the results obtained by carrying out various tests after carrying out.

As is clear from Table 12, in No. 19 which is a conformity example, satisfactory strength, toughness, embrittlement characteristics during use, hydrogen attack characteristics and hardness characteristics, and hydrogen crack resistance characteristics were obtained. .

On the other hand, in No. 20, the temperature between passes is below the lower limit and HAZ
No mechanical property investigation was conducted because delayed cracking occurred in the part. Further, Nos. 21 to 23 were all outside the scope of the present invention, and as shown in the remarks of Table 12, good characteristics were not obtained.

Example 4 Steel A was subjected to narrow groove 1-layer 1-pass multilayer welding under welding conditions WC2 using wire WR4 and various fluxes shown in Table 4.

Table 13 shows the chemical composition of the obtained weld metal.

Then, for the weld metal obtained, heat treatment HT3 shown in Table 6
Table 14 shows the results obtained by carrying out various tests after carrying out.

As is clear from Table 14, in No. 24, which is a conforming example, good results were obtained in all of strength, toughness, embrittlement characteristics during use, hydrogen attack characteristics and hardness characteristics, and hydrogen cracking resistance characteristics. However, in No. 25, the basicity of the flux is low, so the oxygen content of the weld metal is high (WM20; 280ppm, WM21; 410pp
m) It was inferior in toughness.

(Effects of the Invention) Thus, according to the present invention, in submerged arc welding of high strength Cr-Mo steel, not only creep strength but also toughness and in-use embrittlement characteristics, and further excellent in hydrogen crack resistance. Weld metal can be obtained.

[Brief description of drawings]

FIG. 1 is a view showing a groove shape in the embodiment, FIG. 2 is a schematic view of step cooling treatment, and FIG. 3 is a weld metal cross-sectional view showing hardness measurement positions.

Claims (1)

[Claims] 1. C: 0.09 to 0.18 wt%, Si: 0.13 wt% or less, Mn: 0.25 to 0.65 wt%, Cr: 1.85 to 3.25 wt%, Mo: 0.85 to 1.15 wt%, V: 0.23 to 0.37 wt% %, S: 0.015 wt% or less, P: 0.020 wt% or less, tensile strength: 60 to 77.5 kgf / mm ² , 0.2% yield strength Steel plate with strength of 42 kgf / mm ² submerged arc welding As a filler wire, C: 0.07 to 0.15 wt%, Si: 0.30 wt% or less, Mn: 0.50 to 1.00 %%, Cr: 2.25 to 3.25 wt%, Mo: 0.85 to 1.15 wt%, V: 0.20 to 0.35 wt %, Nb: 0.03 wt% or less, Ti: 0.005 to 0.02 wt%, Ni: 0.60 wt% or less, N: 0.005 to less than 0.01 wt%, and the balance is a wire having a composition of substantially Fe. using MgO-BaO-SiO ₂ -CaF ₂ based highly basic firing type flux as the flux, preheat and interpass temperature: 175 to 250 ° C., heat input: welded under the conditions of 20~50kJ / cm, followed At least once in the temperature range of 670 ° C or higher for the welded part (1) TP is the submerged arc welding method for high strength Cr-Mo steel characterized by applying heat treatment after welding to meet the 20.20 to 20.50 of formula. Note TP = (T + 273) x (20 + logt) x 10 ^-3 (1) where T: heat treatment temperature after welding (° C) t: heat treatment time after welding (h)