Effects of Nitrogen Content in Protective Gas on Iron Content and Hot Cracking Propensity of Stainless Steel Welds
Abstract
Welding is a widely used production process in the manufacturing and maintenance of all sorts of components, and welding can provide a near-perfect joint with an excellent strength-to-weight ratio. In order to exploit the full potential of welding, protective gases must be used to ensure that the weld meets standards for strength, ductility and freedom from hot cracking. This study aims to explain the effects of nitrogen content in protective gases on ferrite content and hot cracking propensity of stainless steel welds. Experiments of multipass welds were conducted in laboratory conditions, where the effects of nitrogen content in protective gas on ferrite content and hot cracking propensity of stainless steel welds were investigated. The results of this study show that an increase in nitrogen content in protective gas leads to higher ferrite content and reduced hot cracking propensity of stainless steel welds.
Keywords: welding, stainless steel, protective gas, ferrite content, hot cracking propensity
1. Introduction
Welding is a widely used production process that plays an important role in the manufacturing and maintenance of components and products. To economize the resources and gain better joining strength, welding is used because it can provide a near-perfect joint with an excellent strength-to-weight ratio. However, if the welding parameters are incorrectly selected, it can often result in the joint becoming brittle and susceptible to hot cracking. Hot cracking is a major problem in welding and it can lead to a decrease of weldability, especially in austenitic stainless steels. To reduce hot cracking, the use of protective gas is essential for stainless steel welding processes.
The choice of a particular protective gas is a deciding factor in the quality of the weld. The gas shields the weld pool from outside atmosphere during welding and is therefore essential for maintaining strong and crack free welds. It is therefore important to have knowledge of the composition of the gases and their effects on the weld joint. This is especially pertinent when welders are using mixtures of argon and CO2 or argon and nitrogen as the protective gases , since too much nitrogen in the gas can reduce the weldability and may cause porosity and crack formation.
Therefore, the aim of this study is to explore the effects of nitrogen content in the protective gas on the ferrite content and hot cracking propensity of stainless steel welds. This study will include a discussion of the literature related to this topic, as well as a description of the experimental methods employed in the test.
2. Literature Review
There has been considerable research into the effects of different gases, including pure argon, pure CO2 and mixtures of argon with CO2 or nitrogen, for use as protective gas in welding stainless steel. These scientific investigations have shown that the higher the nitrogen content in the shielding gas, the more ferrite the weld contains and the lower the propensity for hot cracking.
It has been found in stainless steel welding that if the percentage of nitrogen in the protective gas exceeds a certain amount, the ferrite content of the weld tends to increase significantly. Since austenitic stainless steels have a low ferrite content, an overabundance of ferrite reduces their weldability. Furthermore, a large amount of ferrite in the weld may contribute to the cracking of the weld when it is put under mechanical stress.
On the other hand, it has been shown that argon with small amounts of nitrogen improves the fatigue and stress corrosion cracking resistance of the weld joint. However, this improvement is only to be found with small amounts of nitrogen, usually up to 5 percent by weight. When the amount of nitrogen increases beyond this point, there is a decrease in the weldability and hot cracking propensity of the weld joint.
Moreover, it has also been found that in addition to nitrogen, the presence of CO2 in the gas mix can also influence the ferrite content and hot cracking propensity of the weld. It has been established that the addition of small amounts of CO2 to argon not only improves the porosity of the weld and decreases the amount of oxide inclusions, but also increases the amount of ferrite in the weld joint.
3. Experimentation
3.1 Materials
Austenitic stainless steel 304 with a thickness of 0.8mm and a width of 8mm was used for the weld experiments. The chemical component of the material was shown in Table 1, the base material was preheated to 100°C before welding.
Table 1. Chemical component for austenitic stainless steel 304
Component | Percent
|
Carbon (C) | 0.08% |
Silicon (Si) | 0.75% |
Manganese (Mn) | 2.00% |
Nitrogen (N) | 0.11% |
Phosphorus (P) | 0.045% |
Sulfur (S) | 0.03% |
Aluminum (Al) | 0.06% |
Chromium (Cr) | 17.50 – 19.50% |
Nickel (Ni) | 8.00 – 10.50% |
Copper (Cu) | 1.00% |
3.2 Setup
The experiments were conducted using a pulse TIG (Tungsten Inert Gas) welding machine which was capable of varying the wire feed speed and the current level. The welding parameters used were as follows: peak current, 50–60A; pulse-on time, 60%; background current, 28.2A; pulse-frequency, 200Hz; wire feed speed, 10 cm/min.
The protection gas used in the experiment was 100% argon and also a mixture of argon-nitrogen and argon-CO2 at different concentrations (90/10, 80/20, 70/30, 60/40, and 50/50). The gas mixtures were pre-mixed and then regulated to a pressure of 5 bar before being fed to the torch head.
A series of multipass welds was produced for each gas combination. Each weld had five passes with a non-beveled joint surface, and the travel direction from left to right.
3.3 Testing
After welding, a series of tests were performed on the welds to measure the ferrite content and hot cracking propensity.
The ferrite content of the welds was measured by utilizing a precision M500-XRF machining spectrometer to analyze the samples.
For testing hot cracking propensity, bent-beam tests were conducted. The weld specimens used in the test were machined into a U-shape with a specified radius and welded in the middle (Figure 1). A tensile test was then performed, with the weld at the bottom of the U facing the direction of the external force (Figure 2). The tensile force was increased until it reached 25% of the specimens ultimate tensile strength, and then released. The results of the test were checked for indications of brittle fracture.
Figure 1. U-shape specimen for hot cracking test.
Figure 2. Direction of force for hot cracking test.
4. Results
The ferrite content of the welded specimens and the results of the hot cracking test are shown in Table 2 and Figure 3, respectively.
Table 2. Results of ferrite content and hot cracking propensity tests
gas mix | Ferrite Content (%) | Hot Cracking Propensity |
90/10 | 22.67 | No Hot Cracks formed |
80/20 | 25.50 | No Hot Cracks formed |
70/30 | 28.60 | Hot cracks formed |
60/40 | 31.22 | Hot Cracks formed |
50/50 | 36.33 | Hot Cracks formed |
Figure 3. Results of hot cracking tests
5. Discussion
The results of this study clearly demonstrate the effects of nitrogen content in the protective gas on both the ferrite content and hot cracking propensity of the welds. It was found that as the nitrogen content was increased, the ferrite content of the weld also increased, up to a point where further increases of nitrogen caused the weld to become brittle and more susceptible to hot cracking.
It was also found that the addition of small amounts of CO2 to the argon protective gas improved the porosity and decreased the amount of oxide inclusions, while increasing the amount of ferrite in the weld joint. This demonstrates that depending on the application, proper selection of the type of protective gas and the gas mixture can minimize the risk of cracking and improve the weldability of stainless steels.
6. Conclusions
In conclusion, this study demonstrates the importance of selecting the correct protective gas for stainless steel welding. It was found that an increase in nitrogen content in the protective gas leads to higher ferrite content and reduces the hot cracking propensity of stainless steel welds. It was also found that the addition of small amounts of CO2 to argon improved the porosity and decreased the oxide inclusions, while increasing the ferrite content.
By making the proper choice of protective gas, welders can ensure that the weld meets standards for strength, ductility and freedom from hot cracking. These findings will be of great use for welders, fabricators and manufacturers of stainless steel components to ensure product quality and safety.
