Vacuum brazing - a high-quality joining process (Part 3) Brazing apllications
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Stainless steel
Typical applications for the vacuum brazing of stainless steel products are;
- heat exchangers,
- cooler-und heating plates,
- vacuum chambers,
- injection molds,
- impellers for turbo compressors, etc.
- Austenitic (nonhardenable) steels
- Ferritic (nonhardenable) steels
- Martensitic (hardenable) steels
- Precipitation hardening steels, and
- Duplex stainless steels
All these alloys are iron based and contain at least 10% chromium, the basic element that imparts corrosion resistance.
The following material combinations are possible:
- Low carbon, low-alloy, and tool steels
- Nickel-und nickel alloys
- Copper-und copper alloys
- Super alloys
- Tungsten and molybdenum
When designing a joint where dissimilar base metals are involved, it will be necessary to calculate the joint clearance at the brazing temperature to be used. When using stainless steel AISI 304 or 316 in a vacuum braze process , the low carbon (L)should be used. Standard stainless steel is subject to corrosion after the braze cycle, due to the chromium combining with the carbon. This allows the chromium- depleted iron in the base metal adjacent to the grain boundaries to be attacked by corrosive material.
These L-qualities are much less sensitive for those precipitation of chromium carbides during the cooling of the parts at the end of brazing cycle.
Cold worked products of not heat-treatable stainless steels are in annealed condition after vacuum brazing.
A primary requirement for brazing components made of hardenable stainless steel alloys is to use a brazing thermal cycle compactable with the heat treatment required by the alloys.
Precipitation-hardening stainless steels must be brazed at their solution treatment temperature and hardened afterwards by a precipitation treatment at lower temperature.
The stress that can occur in a vacuum brazed turbo compressor impeller from precipitation hardening stainless steel reach the yield strength of base material.The tensile strength of this vacuum brazed joints are around 1120 MPa.
The corrosion resistance of the brazed joints depends on the combination of base material, brazing filler metal, the brazing process, the service conditions and environment. Austenitic stainless steels brazed with nickel-based brazing filler metals or based on precious metal have good heat- and corrosion characteristics. It is thereby important the that the brazing gap it is completely filled to prevent intergranular corrosion.
Carbon steel
Carbon steels can be divided into:
- Low carbon steels
- Low alloy steels
- Heat treatable steels
- Free machining steels
- Tool steels
Vacuum brazing of carbon- and low alloy steels is widely used on a large scale in all market segments. Mass parts for the automobile industry or mechanical engineering are often brazed in continuous furnaces under a reducing gas atmosphere for economic reasons, but dimensions and part weights are most a restrictive factor.
In vacuum furnaces very small parts and large parts, up to 500 kilogram can be brazed. For carbon steels copper based filler materials are mostly used. For high alloy- and tool steels nickel based filler materials are more common.
Carbon steels can be brazed on many other materials like stainless steel, cast iron, tungsten carbide and copper. In some cases an additional thermal treatment can be necessary. This is mostly combined with the braze process.
Typical applications for brazed low-carbon and low-alloy steel are components for automobiles, trucks bicycles, motor cycles and hydraulic components. Other common brazements include turbo compressor wheels, diaphragms, injection moulds.
Treatment processes like nitriding, carbonitriding, thermal galvanizing, chemical or electrolytic nickel plating must be accomplished after the braze process.
Cast iron
In 1969, during a study of the manufacturing of exhaust valve housings for industrial diesel engines the idea was launched to braze cast iron in a vacuum furnace. The basis of this idea was a production cost reduction of approximately 30%. Because the valve housings can be manufactured now from several components, the cast parts are more simple and different materials can be combined.
In the beginning it was difficult to produce reproducible products. The composition and quality of the cast iron was a key factor for obtaining a constant quality. An extensive research program was performed to determine the mechanical properties of vacuum brazed joints between different types of cast iron and in combination with different steels.
The following technological characteristics were examined;
- Tensile- and yield strength at 20 °C and 600 °C
- Shear strength
- Fatigue strength by means of rotation bending test
- Hardness and structure of the base materials.
A disadvantage is however that when cast iron is heated up above 400°C a permanent volume increase arises by transformation in the microstructure of cementite in ferrite and graphite.
This growth is dependent on the chemical composition of the cast iron. The elements Chrome and Manganese stabilize the cementite and minimize this growth. By this uncontrollable change of measure butt joint have an advantage in relation to pin-hole constructions. The main application in brazing cast irons is still the valve housings, about which still thousands are brazed each year.
Nickel- and super alloys
Brazing is widely used to join nickel- and super alloys. These materials are often used in gas turbines because of their good oxidation resistance and their good mechanical characteristics at high temperatures.
A super alloy’s base alloying element is usually nickel, cobalt, or nickel-iron. Oxidation or corrosion resistance is provided by elements such as aluminium, titanium and chromium.
Products are often made of thin plates in order to obtain strong and lightweight constructions. Repair brazing is a cost-effective method for nickel- and cobalt-base super alloys in the aerospace- and gas turbine industry. When repairing heavy contaminated parts it is necessary to clean these before brazing by a treatment in a hydrogen fluoride atmosphere. By this atmosphere, the parts’ surface area becomes depleted of aluminum and titanium so that oxides of these metals do not tend to reform during subsequent operations, such as braze repair.
Nickel – and cobalt base braze materials results in the highest heat resistance and best mechanical characteristics at higher temperature . Beside turbine blades also very thin-walled honeycomb constructions are brazed, like honeycombs seals in gas turbines. Also in the petrochemical industry, many products of super alloys are brazed.
Brazed heater head Stirling Engine
Copper and copper alloys
Copper and his alloys are selected for many braze applications particularly in the electronic and electrical industry, because of their high thermal capacity, good electrical conductivity and excellent heat transfer. Copper is difficult to weld, while brazing can readily be performed on most copper alloys with the proper precautions. The most important impurity in the copper is oxygen. For the brazing process is it important to know which copper quality must be brazed. After the braze process copper is fully annealed.
Copper alloys containing zinc, lead or manganese cannot be vacuum brazed because of the high vapour pressure of these elements. Elements such as chrome, zirconium or beryllium are added to copper to enhance mechanical properties.
As a result of the braze cycle, these properties are lost and difficult to repair Silver-based brazing filler metals are used most commonly in the brazing of copper and copper alloys. Copper can be brazed in combination with stainless steel, steel, ceramic, tantalum and graphite.
Remaining materials
For the vacuum brazing of aluminum, special brazing filler materials alloyed with magnesium are developed. Magnesium can be introduced into the process as elemental magnesium in a tray in the furnace, using magnesium-bearing base metals or using magnesium in the brazing filler metal.
On the brazing surface a oxide layer is always present, which prevents wetting and flowing of the braze material. Because aluminum expands three times as much as aluminum oxide, the oxide layer will crack when heating up the furnace. Above 400 °C the furnace is filled with magnesium vapour, which works as „getter “for the present oxygen. Due to the high vacuum pressure and that low oxygen partial pressure the underlying aluminum does not oxidize again.
The brazing process is very sensitive for small process deviations and is accomplished in special developed vacuum furnaces. Aluminum brazing filler metals for vacuum brazing are alloyed with silicon and magnesium with a brazing temperature of about 560 °C. Furnace brazing in vacuum of aluminum parts is used primarily for the fabrication of heat exchangers.
Other materials which can be brazed successfully in vacuum are: Titanium alloys, tantalum, tungsten carbides, ceramics, diamond, high-speed steels and powder metallurgical materials.
Summary
Vacuum brazing is a high quality joining process and is applied for the following reasons.
- Reproducible joining processes with high mechanical characteristics.
- High alloyed steels which are difficult to weld can be brazed.
- This joining technique offers excellent possibilities to produce complex structures with parts of different materials and to introduce joints on places which are inaccessible by means of other joining techniques, for instance, moulds with cooling channels.
- Immediately after brazing, the products are metallically bright and clean.
- Parts with very thin wall thickness and thick-walled parts can be connected.
- Brazed joints can be well examined by means of non-destructive testing methods.
- Thanks to uniform heating and cooling, deformation is prevented or remains to a minimum.
- The joints are without gaps, hence aseptic, especially important for the food and pharmaceutical industry.
- The brazing temperature can be selected in such a way that for many products brazing and hardening can be combined in one single operation.
- Finally and also very important, this joining technique lead in many instances to a lower cost as compared to other joining techniques.
Brazing Consultancy Peter Steege
