GB1573246A - Submerged arc welding process - Google Patents

Description

(54) SUBMERGED ARC WELDING PROCESS (71) We, SUMITOMO METAL INDUSTRIES, LTD., a Japanese corporation, of No. 15, 5-chome, Kitahama, Higashi-ku, Osaka-shi, Osaka-fu, Japan, do hereby declare the invention, for which we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to a submerged arc welding process and more particularly to a submerged arc welding process suitable for structural members which are subjected in use to low temperatures. More particularly, the present invention relates to a submerged welding process in which weld metal is deposited in a plurality of superimposed layers.

Conventionally, vessels for storing liquefied gas such as liquefied nitrogen, liquefied oxygen or the like are constructed from steel material by means of welding. Since these vessels are subjected in use to low temperatures such as below minus 100"C, it is very important to provide an adequate impact-resistant property at such low temperatures.

It has been found, however, that conventional submerged arc welding processes have not been satisfactory for structures which are subjected in use to low temperatures particularly in respect of the toughness of the weld metal. Such conventional submerged arc welding processes are conducted with a relatively low thermal input which would be expected to be effective to produce fine crystalline structure having a satisfactory impact resistance. However, it has not been possible to provide an adequate impact resistance at low temperatures only through a relatively low thermal input.

According to the present invention, the above and other objects can be accomplished by a submerged arc welding process in which use is made of submerging flux having a basicity as defined by a formula CaO +MgO SiO2 in weight percentage of between 1.5 and 3 and which comprises the step of producing a plurality of superimposed deposited layers of weld metal under a welding current of 400 to 700A and an arc voltage of 35 to 48V, each of said layers having a thickness not greater than 7 mm so that the weld metal in an underlaying layer is thermally affected by an adjacent overlying layer whereby recrystallisation is effected in a substantial thickness of the underlying layer.

Such a process is particularly suitable for use in constructing vessels for use at extremely low temperatures.

It is preferred to use a bond type flux including on the basis of weight 10 to 30 percent of SiO2, 8 to 20 percent of Al,03, 25 to 45 percent of MgO, 10 to 30 percent of CaO, 7 to 20 percent of CaF2 and at least one material selected from metallic Si, Fe-Si and Fe-Si-Mn in an amount of 0 to 0.6 percent calculated in term of metallic Si. Where the welding process is applied to a steel material containing a relatively high percentage, for example 3.5% of nickel, it is also preferred to use a cored wire welding electrode consisting of a mild steel tube and core material, the latter containing on the basis of total electrode weight 5 to 25 percent of CaF2, 2.5 to 5.5 percent of Ni, 0 to 0.5 percent of Mo, 0 to 0.5 percent of Ti.

It is recommended to maintain the thermal input below 40,000 J/rm.

The present invention is based on the findings that, in a welding structure having a plurality of superimposed layers of weld metal, an underlying layer receives a thermal influence from an adjacent overlying layer when the latter is being deposited so that the former is caused to produce a recrystallised fine structure and that the fine structure can effectively be utilised in obtaining an improved impact resistance at low temperatures such as below minus 1000 C. Thus, when the weld metal is deposited in relatively thin superimposed layers only a relatively thin portion in each layer remains thermally unaffected. Such thermally unaffected portion in each layer is preferably less than 2 mm so that a substantial part of the layer is occupied by the recrystallised metal of high impact resistance.

In order to obtain a high impact resistance or an improved toughness, it is required to maintain the oxygen content in the weld metal below 300 ppm., and this is accomplished through the use of flux in which the said basicity is greater than 1.5. Further, a higher basicity is effective to decrease the Si content in the weld metal. However, since an increase in the basicity has an adverse effect on the weldability, the value should not exceed 3.0.

With respect to the flux, the aforementioned composition is recommendable for welding steel material containing high percentages of nickel for the following reasons.

The SiO2 has an influence on the melting point of the flux and where the SiO, content is less than 10 percent there will be an increase in the melting point of the flux so that adverse effects will be seen in the performance of the welding operation and also in the appearance of the welded beads. With the SiO, content exceeding 30 percent, the SiO, will be chemically reduced and there will be an increase in the Si content in the weld metal resulting in a poor toughness of the weld metal.

The A1203 has an influence on the appearance of the welded beads and an acceptable range is between 8 and 20 percent. With the MgO content less than ZS percent, it will be difficult to maintain the basicity at a desirable level but where the content is greater than 45 percent the melting point of the flux will be increased to an unacceptable level. The CaO content should be greater than 10 percent in order to maintain the basicity within the desired range but it will have an adverse effect on the workability if it is increased beyond 30 percent.

The CaF2 content should be greater than 7 percent in order to provide a satisfactory appearance on the welded beads. However, excessive addition of CaF2 causes an unstable welding arc so that the content should be lower than 20 percent. In order to maintain the silicon content in the weld metal below 0.20 percent, it is required to maintain the silicon content of metallic Si, Fe-Si and Fe-Si-Mn in the flux to lower than 0.6 percent. Otherwise, there will be an adverse effect on the toughness at low temperatures due to an increase in the silicon content in the weld metal. As mentioned above, silicon containing deoxydizing agent may be metallic Si, Fe-Si, Fe-Mn-Si.

It is of course possible to use a material other than silicon as the deoxydizing agent.

For example, manganese may be used for the purpose. The flux may contain a deoxydizing agent in an amount of less than 0.6% calculated in term of metallic silicon.

Regarding the cored wire, it has been found that the CaF2 content should be greater than 5 percent to the total weight of the wire. Otherwise, blow-holes are apt to be produced in the weld metal and there will be a decrease in the toughness. With the CaF2 content greater than 5 percent, there is a remarkable decrease in the oxygen content in the molten metal so that blow holes are prevented and the toughness is improved. However, the CaF, content should not exceed 25 percent because an excessive CaF2 content makes the welding arc unstable and cause a poor workability.

In order to ensure an adequate impact-resistant property at minus 1000C, it is desirable to maintain the nickel content in the cored wire in an amount of higher than 2.5 percent; however, where the content increases beyond 5.5 percent, it may cause cracks under high temperature.

Molybdenum may be incorporated to the cored wire for obtaining an increased strength of the weld metal but the content shall not exceed 0.5 percent because it may have an adverse effect on the impact-resistant property at low temperatures where the Mo content is above this value.

Titanium may be also incorporated in the cored wire because it is effective to produce fine crystalline structures which serve to provide an improved impact-resistant property under low temperature. However, the titanium may be omitted because even when no titanium is present it is possible to obtain a satisfactory impact-resistant property around minus 100"C. Where the Ti content is greater than 0.5 percent, there will be a decrease in the toughness due to an increase in the silicon content in the weld metal.

Nickel, molybdenum and titanium in the core material may be incorporated in the welding electrode in the form of ferrous alloy thereof, for example, Fe-Ni, Fe-Mo and Fe-Ti. Fe-Ni, Fe-Mo or Fe-Ti may be incorporated in the electrode in the abovementioned amount calculated in term of nickel, molybdenum or titanium. Of course, nickel, molybdenum or titanium may be added to the electrode in the form qf elementary metal.

In the preferred form of the present invention, the process is performed with a thermal input less than 40,000 J/cm. With the welding current less than 400A, it becomes difficult to maintain a stable welding arc, while the welding current exceeding 700A produces deposited layers having a thickness greater than the desirable value.

With the arc voltage less than 35V, there will be an increase in the melting rate of the weld metal while a voltage more than 48V causes an unstable welding arc. The limit of the thermal input is necessary in order to maintain the fine crystalline structures and the thin deposited layers of the welding metal.

The inventors also had found that under the aforementioned welding condition of the present invention, welding should be preferably conducted at a velocity of 20 to 50 cm/minute to provide deposit layer of a thickness less than 7 mm.

The features of the present invention will become more apparent from the following description of process embodying the invention, with reference to the accompanying drawings, in which: Figure 1 is a fragmentary perspective view showing steel plates which are being welded by a process embodying the present invention; Figure 2 is an enlarged fragmentary sectional view showing an example of deposited layers of weld metal; Figures 3A, B and C show examples of deposited layers or beads of the weld metal; and Figure 4 is a sectional view showing an example of a welding groove formed in the material to be welded.

Referring to Figure 1, a pair of plates 1 and 2 of nickel-containing steel such as 3.5% nickel steel are placed in end-to-end relationship with a substantially U-shaped groove 3 formed at the abutting portion. A submerged arc welding is performed under the aforementioned welding conditions using the aforementioned flux and the aforementioned welding metal to form a plurality of deposited layers 4 which are positioned in superimposed staggered relationship. The thickness T of each deposited layer 4 is less than 7 mm.

Referring specifically to the layer 4a shown in Figure 1, it receives a thermal effect at the area designated by the numeral 5 when the adjacent layer 4b is formed so that recrystallization proceeds in the area 5 to produce a fine crystalline structure.

Further, when another adjacent layer 4c is deposited the metal in the layer 4a receives a thermal effect at the area designated by the numeral 6 so that a fine crystalline structure is produced in the area 6. Since the layer 4a is relatively thin, the layer is substantially occupied by the recrystallized fine structure and the thermally unaffected layer is 2 mm thick or less. Thus, it is possible to produce a weld structure having a high impact resistance at extremely low temperatures. Figure 2 shows the deposited layers 4 of the weld metal in detail. In the drawing, the numeral 8 shows the area where the metal has received a thermal effect and recrystallization has proceeded, the areas 7 being thermally unaffected.

Referring now to Figure 3, there are shown several examples of deposited layer arrangements. More specifically, Figure 3A shows an example wherein layers 4 of the weld metal are deposited one on another without any transverse offset, while Figure 3B shows an example of an arrangement similar to Figure 2, in which successive layers are deposited with transverse offsets alternating in direction. It will be noted that the latter arrangement is preferable in respect of recrystallization. In Figure 3C, the layers are also in a staggered arrangement.

Examples.

Welding operations were performed on 3.5% Ni steel meeting the specification of ASTM A-203 and having the dimension as shown in Figure 4. In performing the welding processes, flux materials have been prepared as shown in Table I.

TABLE I CaO MgO SiO2 Al2O3 CaF2 Basicity A 20 29 38 10 3 1.3 B 20 25 25 20 10 1.8 C 16 26 21 20 17 2.0 D 16 37 21 14 12 2.5 Note: The basicity is defined by the CaO + MgO formula B = (in weight %) SiO2 Further, the welding electrode contained on the basis of total weight 10 percent of CaF2, 2.7 percent of Ni, 0.5 percent of Mo and the balance of Fe a mild steel.

The mild steel had a composition of by weight, 0.05 percent of carbon, 0.50 percent of manganese and the balance of iron. The welding operations were performed as shown in the Table II and the welded specimens were formed with U-shaped notches of 2 mm width at the welded portions and subjected to Charpy impact tests at a temperature of minus 101 C. The results are also shown in Table II.

TABLE II Deposited Test Results Welding Condition Layers Oxygen Content Thermal Thickness Thermally in Welding Impact Current Voltage Input of Layer Unaffected Metal Resistance Specimen Flux (A) (V) (J/cm) (mm) Layer (ppm) (kg m)

0000 Q00060 N NNNN o m o o o o o o o m o m o N a > t N N N N > > n > N t m 450 28 > X O 1 m t t~ 2 N > 30 o o H ~ H < o o - 3 D 800 45 45000 9-10 2.5 250 1.8 4 D < t 45000 m m 2.7 230 0.9 o I I I H oo o m e t t m u) B B s 8 g 8 o 8 8 s o 3 Q 0 0 0 0 0 0 a 7 D t N 30000 N n N > 8 500 t oo 0t > 1.0 250 8.3 9 D 550 45 35000 5-6 1.2 250 9.5 0 a . 10 D 600 45 35000 5-6 1.5 275 6.9 0 a 11 B 650 45 37000 6-7 1.7 260 7.8 o D 420 38 25000 o o 245 10.4 oooo t sD 00 co 25000 m m X - d a a a a < : a a U a a m a a H cs t m o > oO as ~ < H X saldwexg uolluaAuI aAIlese dwo D In the above tests, it was found that the specimens 1 and 2 had relatively thick beads of cross-section having relatively small radius of curvature at the surface of each layer. This is caused by an insufficient arc voltage. Thus, there was a relatively large area of thermally unaffected metal. As the results show, they had very poor impact resistance.

In the specimens 3 and 4, it will be seen that excessive welding current and thermal input have produced deposited layers of excessive thickness. Therefore, it has not been possible to produce an adequate coverage of recrystallised fine structures.

The specimen 5 was welded using the flux of lower basicity. Therefore, there was an excessive amount of residual oxygen in the welded metal. The specimens 6 to 13 which have been welded in accordance with the present invention had thermally unaffected areas of less than 2 mm thick. Thus, these specimens had a satisfactory impact resistance.

Claims (6)

WHAT WE CLAIM IS:

1. A submerged arc welding process in which use is made of a submerging flux having a basicity as defined by a formula CaO +MgO SiO2 in weight percentage of between 1.5 and 3 and which comprises the step of producing a plurality of superimposed deposited layers of weld metal under a welding current of 400 to 700A and an arc voltage of 35 to 48V, each of said layers having a thickness not greater than 7 mm so that the weld metal in an underlying layer is thermally affected by an adjacent overlying layer whereby recrystallisation is effected in substantial thickness of the underlying layer.

2. A submerged arc welding process in accordance with claim 1 in which said deposited layers of weld metal are arranged in staggered relationship.

3. A submerged arc welding process in accordance with claim 1 or 2 for use in welding nickel-containing steel, said process using a bond-type flux containing on the basis of weight 10 to 30 percent of SiO2, 8 to 20 percent of Altos, 25 to 45 percent of MgO, 10 to 30 percent of CaO, 7 to 20 percent of CaF2 and at least one material selected from the group consisting of metallic Si, Fe-Si, Fe-Si-Mn in an amount 0 to 0.6 percent calculated in term of metallic Si.

4. A submerged arc welding process in accordance with claim 3, in which use is made of a welding electrode of cored wire consisting of a mild steel tube and core material, the core material including, on the basis of weight to the total weight of the welding electrode, 5 to 25 percent of CaF2, 2.5 ro 5.5 percent of nickel, 0 to 0.05 percent of Mo, 0 to 0.5 percent of Ti.

5. A submerged arc welding process in accordance with claim 1, in which thermal input is maintained below 40,000 J/cm

6. A submerged arc welding process, as claimed in claim 1 and substantially as herein described with reference to the accompanying drawing.