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The basics of stainless steel braze welding
In this article, we are going to see a type of brazing that concerns the joining of very different metals such as copper and stainless steel.
We recommend you read the article regarding copper-to-copper brazing because much information is common to the two applications.
In this brief guide, we will consider brazing that is done primarily in the refrigeration industry, when stainless components need to be connected to copper elements.
Specifically, we will see the steps required to be able to perform a copper brazing with stainless steel by flame.
As we already know, through brazing it is possible to join materials of different types by avoiding their melting through the use of a metal alloy that has the binding function of creating a solid and permanent joint. Let’s delve into the processes for strong brazing of copper with stainless steel.
Alloys and fluxes for steel brazing
Because stainless steel exhibits surface oxidation that is quite difficult to remove, it is necessary to use specially designed fluxes to provide a high level of protection for stainless steel during brazing operations. They also help to remove the present oxide.
There are mainly two approaches depending on the type of base materials:
- brazing with bare alloy and separate paste flux
- brazing with coated alloy, with flux already included in the rod.
In the first case, the brazing area can be protected by applying flux with a clean brush and then proceeding with heating and adding the filler material.
This solution has the advantage of protecting the area very well, but it is less practical and time-consuming.
In addition, the flux must be specific to the application otherwise there is a risk that it will not be strong enough or aggressive enough against the oxides formed during the process.
If, on the other hand, the coated alloy approach is chosen, we recommended products with a sufficient amount of flux to ensure good joint coverage by casting from the rod into the affected area.
The advantage of this solution is the speed of execution, while it comes with a little less flexibility on the choice of flux. Often, in fact, the flux on the rod is of a generic type and not specific to stainless steel.
Based on our experience you still manage to get good results and achieve a good compromise between quality and productivity.
From a practical point of view, before heating the joint, you should place the rod on the joint and drop some flux on the part by bringing the flame closer until the coating becomes liquid. In this way, a protective layer can be applied to the affected area before the actual brazing.
In addition, to prevent overheating of stainless steel, it is recommended to use high-silver alloys such as ISO alloy 17672: 2016 Ag 145 – Ag 155 – Ag 156.
These alloys have a low melting temperature and allow easy brazing.
References can be found in the literature to possible corrosion risks of joints brazed with silver-based and zinc-containing alloys when exposed to moisture. (Based on our many years of experience, however, we have had no such findings).
Going back to the literature, to avoid this risk, the use of Zinc-free alloys is mentioned. In practice, these types of alloys are very expensive, so the use of the above alloys is preferred.
If you have any questions about product choice according to the type of application, our technical support is at your disposal.
Designing and preparing the joint for brazing
Given the differences between the two materials, when designing, it is useful to consider the assumptions made earlier and leave space for the operator to be able to heat the joint properly and easily apply the flux.
Flame adjustment for iron brazing
A flame with a “neutral” setting, so not too aggressive, is recommended for brazing steel.
The main reason lies in the different thermal conductivity of the two materials: stainless steel has a conductivity of 17 W-m-1-K-1 (watts per meter-kelvin) while the value for copper is 390 W-m-1-K-1 (watts per meter-kelvin).
What to avoid
In fact, if we start heating the joint by aiming the flame at the stainless steel we will get it heated (steel retains a lot of heat), while the copper will remain cold.
In this case, when the molten brazing alloy reaches the copper component, finding it insufficiently hot, it will stop. If instead we shift the heating to the copper part, we will get excessive heating of the steel part, which will make brazing in general difficult.
In addition, many oxides will form due to too long a heating time, and this may block the penetration of the alloy (poor wettability).
Correct procedure
Therefore, excluding cases where the piece of stainless steel is much larger (greater mass to be heated), it is recommended to first proceed with heating the copper and, through the same (due to its excellent conductivity), get to heat the stainless steel and then proceed with the addition of brazing material.
VIDEO: stainless-steel brazing technique
Once the products to be used have been defined and the pieces to be brazed have been thoroughly cleaned, the flame can be ignited and properly adjusted.
Then proceed with the approach of the flame, taking care not to dwell on one spot, but moving it along the affected area.
At the same time, it is important to coat the joint to be brazed by resting the rod occasionally to check whether the flux contained becomes liquid by wetting the joint, or observing when the paste flux becomes transparent.
Once the flux becomes liquid you can add the filler metal being very careful not to aim the flame directly at the brazing alloy, but melting it by conduction.
Once the melting of the alloy is achieved, the flame should be used to turn around the joint so that the filler material follows the heat and fills the joint.
Conclusions
Stainless steel brazing has many similar characteristics to applications already seen, such as copper brazing. However, there are a number of tricks related to the choice of alloys, flux, and heating of the joint that require specific knowledge and experience.
Brazing processes can be automated with the aid of preformed products like rings/preforms, which are designed and produced based on the geometry of the joint to be produced.
Saldobrase’s experts are available to support you in choosing and studying the best brazing process according to your specific needs.
