1. Brazing performance
The main problem in brazing stainless steel is the presence of oxide films on the surface, which significantly affects the wettability and spreading of the filler metal.
Various stainless steels contain a considerable amount of Cr, and some also contain elements such as Ni, Ti, Mn, Mo, Nb, etc., which can form various oxides and even complex oxides on the surface. Among them, Cr and Ti oxides, Cr 2 Ó 3 and TiO 2 are quite stable and difficult to remove.
When welding in air, an active flux is needed to remove them. When brazing in a protective atmosphere, the oxide film can only be reduced under low dew point, high purity atmosphere and sufficiently high temperature. In vacuum brazing, high vacuum and temperature are required to obtain good brazing effects.
Another problem in brazing stainless steel is that the heating temperature has a significant impact on the microstructure of the base material. The heating temperature for brazing austenitic stainless steel should not exceed 1150°C, as excessive grain growth may occur.
If the austenitic stainless steel does not contain stabilizing elements such as Ti or Nb and has a high carbon content, brazing within the sensitization temperature range (500-850°C) should be avoided to avoid precipitation of chromium carbides and reduce the corrosion resistance.
The temperature selection for brazing martensitic stainless steel is more strict. It requires that the brazing temperature matches the quenching temperature, combining the brazing process with the heat treatment process, or the brazing temperature must be lower than the tempering temperature to avoid softening the base material during brazing.
The temperature selection principle for brazing precipitation hardened stainless steel is the same as that for martensitic stainless steel, where the brazing temperature must match the heat treatment regime to obtain optimal mechanical properties.
In addition to the two main problems above, austenitic stainless steel is prone to stress cracking during brazing, especially when brazed with copper-zinc filler metals. To avoid stress cracking, the workpiece must undergo stress relief annealing before brazing, and the heating of the workpiece during brazing must be as uniform as possible.
2. Brazing Materials
(1) Filler metals
According to the requirements of stainless steel welding, the commonly used filler metals for stainless steel brazing include tin-lead solder, silver-based filler metals, copper-based filler metals, manganese-based, nickel-based filler metals and precious metals. filler metals.
Tin-lead solder is mainly used for soft brazing of stainless steel, and a higher tin content is preferred. The higher the tin content in the filler metal, the better its wetting ability on stainless steel.
The shear strength of 1Cr18Ni9Ti stainless steel joints welded with various commonly used tin-lead solders is listed in Table 3. Due to the low strength of the joint, it is only used for brazing parts with low load-bearing requirements.
Table 3 Shear strength of 1Cr18Ni9Ti stainless steel joints welded with tin-lead solder
| Solder Alloy Class | Sn | S-Sn90Pb | S-Pb58SnSb | S-Pb68SnSb | S-Pb80SnSb |
| Shear force /MPa |
30.3 | 32.3 | 31.3 | 32.3 | 21.5 |
Silver-based filler metals are the most commonly used filler metals for brazing stainless steel, with silver-copper-zinc and silver-copper-zinc-cadmium filler metals being the most widely used due to their impact minimum in the properties of the base material during brazing. temperatures.
The strength of 1Cr18Ni9Ti stainless steel joints welded with various commonly used silver-based filler metals is listed in Table 4. Stainless steel joints welded with silver-based filler metals are rarely used in highly corrosive environments, and the operating temperature of the joints generally decreases. do not exceed 300°C.
When brazing nickel-free stainless steel, a filler metal with higher nickel content such as B-Ag50CuZnCdNi should be used to prevent corrosion of the welded joint in humid environments.
When brazing martensitic stainless steel, a filler metal with a brazing temperature not exceeding 650°C, such as B-Ag40CuZnCd, should be used to prevent softening of the base material.
When brazing stainless steel in a protective atmosphere, a lithium-containing self-fusing filler metal such as B-Ag92CuLi and B-Ag72CuLi can be used to remove the oxide film from the surface.
When brazing stainless steel in a vacuum, a silver filler metal containing elements such as Mn, Ni and Rd can be selected to ensure good wetting properties of the filler metal even without easily evaporating elements such as Zn and Cd.
Table 4 Resistance of 1Cr18Ni9Ti stainless steel joints welded with silver-based filler metals
| Brazing Material Class | B-Ag10CuZn | B-Ag25CuZn | B-Ag45CuZn | B-Ag50CuZn |
| Tensile strength /MPa |
386 | 343 | 395 | 375 |
| Shear force /MPa |
198 | 190 | 198 | 201 |
| Brazing Material Class | B-Ag70CuZn | B-Ag35CuZnCd | B-Ag40CuZnCd | B-Ag50CuZnCd |
| Tensile strength /MPa |
361 | 360 | 375 | 418 |
| Shear force /MPa |
198 | 194 | 205 | 259 |
The main copper-based brazing materials used for brazing different steel joints are pure copper, copper-nickel and copper-manganese-cobalt brazing materials. Pure copper brazing material is mainly used for brazing under gas shielding or vacuum conditions.
It is suitable for stainless steel joints with a working temperature of no more than 400 ℃, but its oxidation resistance is not good. Copper-nickel brazing material is mainly used for flame brazing and induction brazing.
The strength of the 1Cr18Ni9Ti stainless steel welded joint is shown in Table 5. It can be seen that the joint has the same strength as the base material and can work at higher temperatures.
Copper-manganese-cobalt brazing material is mainly used for brazing martensitic stainless steel in a protective atmosphere. The strength and working temperature of the joint can be comparable to that of the joint welded with gold-based brazing material.
For example, the performance of the 1Cr13 stainless steel joint welded with B-Cu58MnCo brazing material is equivalent to that of the same stainless steel joint welded with B-Au82Ni brazing material (see Table 6), but the production cost is greatly reduced .
Table 5 Shear strength of 1Cr18Ni9Ti stainless steel joint welded with high temperature copper-based brazing material.
Table 5 Shear strength of 1Cr18Ni9Ti stainless steel joint welded with high temperature copper-based brazing material.
| Brazing Material Class | Tensile strength /MPa |
||
| 20°C | 400°C | 500℃ | |
| B-Cu68NiSiB | 324~339 | 186~216 | – |
| B-Cu69NiMnCuSiB | 241~298 | – | 139~153 |
Table 6 Shear strength of 1Cr13 stainless steel welded joints
| Brazing Material Class | Tensile strength /MPa |
|||
| Room temperature | 427°C | 538°C | 649°C | |
| B-Cu58MnCo | 415 | 217 | 221 | 104 |
| B-Au82Ni | 441 | 276 | 217 | 149 |
| B-Ag54CuPd | 399 | 207 | 141 | 100 |
Manganese-based brazing materials are mainly used for gas shielded brazing, and high purity gas is required. To avoid grain growth in the base material, it is advisable to use brazing temperatures below 1150°C with the corresponding brazing material.
Satisfactory brazing results can be achieved with stainless steel joints welded with manganese-based brazing materials as shown in Table 7. The joint can withstand working temperatures of up to 600°C.
Table 7 Shear strength of 1Cr18Ni9Ti stainless steel joints welded with manganese-based brazing material.
| Brazing Material Class | Tensile strength /MPa |
|||||
| 20°C | 300°C | 500℃ | 600°C | 700°C | 800°C | |
| B-Mn70NiCr | 323 | 一 | 一 | 152 | – | 86 |
| B-Mn40NiCrFeCo | 284 | 255 | 216 | – | 157 | 108 |
| B-Mn50NiCo | 325 | 一 | 253 | 160 | – | 103 |
| B-Mn50NiCuCrCo | 353 | 294 | 225 | 137 | – | 69 |
| B-Mn52NiCuCr | 366 | 270 | 一 | 127 | – | 67 |
When nickel-based solder is used for brazing stainless steel, the joint performs well at high temperatures. This type of solder is generally used for gas shielded brazing or vacuum brazing.
To overcome the problem of excessive formation of brittle compounds in the welded joint, which leads to a significant reduction in the strength and plasticity of the joint, the gap of the joint should be minimized as much as possible, and the elements that easily form brittle phases in the welded joint solder must be fully diffused into the base material.
In order to avoid the phenomenon of grain growth in the base material at brazing temperature due to excessive insulation time, short-term insulation and diffusion treatment at low temperature (compared with brazing temperature) can be adopted after brazing. brazing.
Precious metal solders used for brazing stainless steel mainly include gold-based solders and palladium-containing solders, among which the most typical are B-Au82Ni and B-Ag54CuPd. B-Au82Ni has good wettability.
The stainless steel welded joint has high high temperature resistance and oxidation resistance, and the maximum working temperature can reach 800 ℃. B-Ag54CuPd has similar characteristics to B-Au82Ni and has a lower price, so there is a tendency to replace B-Au82Ni.
(2) Brazing flux and furnace atmosphere: The surface of stainless steel contains oxides such as Cr 2 Ó 3 and TiO 2 , which must be removed using active flux. When brazing stainless steel with tin-lead solder, phosphoric acid solution or zinc oxide hydrochloric acid solution can be used as flux.
The phosphoric acid solution has a short activation time and requires a rapid heating brazing method. When brazing stainless steel with silver-based solder, FB102, FB103 or FB104 flux can be used. When brazing stainless steel with copper-based solder, FB105 flux is used due to the higher brazing temperature.
When brazing stainless steel in a furnace, vacuum atmospheres or protective atmospheres such as hydrogen, argon, and decomposed ammonia are commonly used. In vacuum brazing, the vacuum pressure should be less than 10-2Pa.
When welding in a protective atmosphere, the gas dew point should not exceed -40°C. If the gas purity is not sufficient or the brazing temperature is not high, a small amount of gas stream such as boron trifluoride can be added to the atmosphere.
3. Brazing Techniques
Before brazing stainless steel, more rigorous cleaning must be carried out to remove any film of grease and oil, and brazing must be carried out immediately after cleaning.
Brazing stainless steel can be done using flame, induction, or oven heating methods. The furnace used for brazing must have a good temperature control system (brazing temperature deviation must be within ±6°C) and be able to cool quickly.
When using hydrogen as a shielding gas for brazing, the hydrogen requirements depend on the brazing temperature and the composition of the base material.
In other words, the lower the brazing temperature and the higher the stabilizer content in the base material, the lower the hydrogen gas dew point will be.
For example, for martensitic stainless steels such as 1Cr13 and Cr17Ni2t, the hydrogen gas dew point should be below -40°C at a brazing temperature of 1000°C; for unstabilized 18-8 chromium-nickel stainless steels, the dew point of hydrogen gas should be below 25 ℃ at 1150 ℃ brazing temperature; but for 1Cr18Ni9Ti stainless steel with titanium stabilizer, the dew point of hydrogen gas at brazing temperature of 1150℃ should be below -40℃.
When using argon as a shielding gas for brazing, higher purity of the argon gas is required.
If copper or nickel plating is applied to the stainless steel surface, the purity requirement of the shielding gas can be reduced.
To ensure the removal of the oxide film on the surface of stainless steel, BF3 gas flux can be added or self-melting solders containing lithium or boron can be used. The vacuum required for vacuum brazing of stainless steel depends on the brazing temperature. As the brazing temperature increases, the degree of vacuum required can be reduced.
The main process after brazing stainless steel is to clean the residual flux and residual flow inhibitor, and, if necessary, carry out post-brazing heat treatment. Depending on the flux and brazing method used, residual flux can be removed by water washing, mechanical cleaning or chemical cleaning.
If abrasive is used to clean residual flux or oxide film in the vicinity of the joint, sand or other fine non-metallic particles should be used.
For martensitic stainless steel and precipitation hardened stainless steel parts, post-brazing heat treatment is required according to the special requirements of the material.
Stainless steel joints welded with nickel-chromium-boron and nickel-chromium-silicon solders are often subjected to diffusion heat treatment after brazing to reduce joint clearance requirements and improve joint structure and properties.
