This issue of Metals is dedicated to recent advances in low-carbon and stainless steel. Stainless steels like the one found in a 5D Bend and low-carbon steels are not new materials, but recent developments have pushed their potential to new heights. In this issue, we’ll explore recent developments in Type 410 autogenously welded steel and in-line cooling-control methods, as well as new materials and processes that make them more cost-effective.
High-speed Autogenously Welded Stainless Steel
Type 410 steel is a martensitic, high-speed autogenously welded stainless steel with a ductile modulus. Its chemical composition is shown in Table 1. The results of typical longitudinal-face bend tests for Type 410 steel are presented in Table 2. Welds with a weld cooling control show better overall ductility. Average longitudinal tensile test results are shown in Table 3.
A scanning electron microscopy study revealed that the AISI 410S steel had undergone a martensitic transformation. The analysis also indicated the formation of deleterious phases such as chromium carbides and precipitates. The results indicate that the advancing side of the FSW welds exhibits higher levels of chromium carbides and other undesirable phases. These results are consistent with previous studies of FSW welds.
When the test was repeated at 450 pm, the heat input dropped to 562.6 J/mm. While these temperatures were lower, the cooling rate increased significantly. Both factors affect the corrosion resistance of AIS 410 steel. While the Ir/Ia ratio is 60% lower than in Condition 1, reactivation peaks occurred in the upper line of the sample in both conditions. The results suggest that Type 410 steel is more resistant to oxidation and corrosion than conventional stainless steel.
The susceptibility of AISI 410S ferritic stainless steel to intergranular corrosion is attributed to the FSW process. The FSW process produces joints with low Ir/Ia values and no evidence of microstructural changes detrimental to corrosion resistance. The speed of rotation increases the susceptibility to intergranular corrosion and precipitation of Cr carbides. Compared to the fusion process, these results indicate that AISI 410S ferritic stainless steel is more resistant to intergranular corrosion than its counterparts.
MSS allows for efficient thermal processing. The weld seam undergoes multiple cooling cycles, resulting in an increase in maximum strain at fracture and a decrease in ultimate tensile strength. The resulting strength is approximately 200 ksi (or 1,400 MPa) – close to the strength of standard structural steel. The thin-wall structural parts undergo complete quenching in gentle air-cooling cycles.
A significant advantage of cooling-controlled welding is that it is more stable and less expensive than conventional processes. The main drawback of this method is that it may cause corrosion, especially in ferritic stainless steels, which are well-suited to caustic environments. As a result, this type of steel is widely used in engineering applications. However, it is not ideal for welding ferritic stainless steels, as the process can cause depletion of chromium in the weld matrix.
In order to achieve a high-quality weld in stainless steel, a process known as cooling-controlled welding requires low diffusible hydrogen electrodes with a designator of H2 or H4, H8, or H16. The H designator indicates the degree of hydrogen in the weld metal. High-temperature welding in high-carbon steels also results in a hard weld zone adjacent to the weld metal, which is susceptible to cracking. Using a small wire brush or chipping tool can help avoid a hard structure in this area.
The welding process used in stainless steel is similar to that of carbon steel but requires preheating. A preheat temperature of at least 200degF is required for stainless steel over 3/4 inch. Thermal chalk is used to monitor the preheat temperature, which melts when the part reaches the required temperature. The preheat time and temperature must be optimized based on the type of welding.
In addition to the benefits of cooling-controlled welding, it is also important to note that austenitic stainless steel alloys contain less than 20% nickel. These alloys are particularly difficult to weld, so alternative methods are needed. A proper understanding of this process will ensure that it is the best way to weld in these materials and that it is less costly in the long run. This type of welding is often used for critical applications, such as in high-temperature steam turbines.
During the welding process, it is necessary to control the interpass temperature to keep it under 400 degrees F. However, this process is not recommended for medium-carbon steels, because the rate of cooling will be too high, leading to a hard structure in the heat-affected zone. The best method of welding stainless steel is the continuous-cooling welding technique. If the metal temperature increases, it can lead to cold cracking, which is a potentially serious problem. Reduce the amount of hydrogen in the weld pool to keep it below the desired temperature.
In-line Cooling Control Materials
Stainless steel, especially the austenitic type, is a valuable material used in a variety of applications. Its outstanding properties include low carbon content, which is less than 0.08% by weight, resistance to elevated temperatures, and low stacking fault energy. It is also nonmagnetic and is known to exhibit dynamic strain aging and the Portevin-Le Chatelier effect.
The stainless characteristic of chromium-containing alloys is obtained by a layer of chromium oxide that forms on exposed surfaces over a short period of time. By removing this oxide layer, the alloy can become carbon-transparent. The process of surface activation is often called depassivation. This method has proven effective for a wide variety of austenitic steels and stainless steels.
Another method for improving the corrosion resistance of low-carbon steels is through the laser powder bed fusion process. This process was developed by optimizing critical process parameters including laser power, scan speed, hatch spacing, and powder layer thickness. The resulting clad layer exhibited greater austenite content, indicating improved corrosion resistance. As a result of the improved corrosion resistance, the clad thickness was increased to 65.8 um.
The use of a good corrosion inhibitor is critical in preventing SCC. Phosphates and chromate have been proven effective in preventing SCC. Stainless steel is susceptible to biofilms formed by microorganisms living in the cooling water. These biofilms contain hydrated polymeric secretions and may contain diverse types of bacteria, from aerobic organisms at the water interface to anaerobic ones on the oxygen-depleted surface of the metal.
A high-temperature solution heat treatment, commonly called solution-annealing, is a technique used to enhance the corrosion resistance of low-carbon and stainless steels. The alloy is heated to a temperature between 1950degF and 2050degF, then cooled using air or water. Stabilizing elements, such as manganese or nickel, have a higher affinity for carbon than chromium, reducing the carbide formation rate of chromium. This method results in the development of high-strength, stabilized grades, such as 321H stainless.
The presence of an active species in water may increase the risk of SCC. Polysilicate ions and colloidal silica are two of the most important species in water at the pH levels that cooling systems maintain. They are formed very slowly from monosilicic acid, which is the predominant species in water. When these ions are present in high concentrations, it can increase the risk of SCC in a metal.
Martensitic Stainless Steels
In order to increase the strength of a material, a process known as hardening is applied. During this process, the steel is heated to temperatures above 980 degC. The steel then undergoes a process known as quenching, in which the temperature is dropped rapidly. The process converts the austenite crystals to martensite, which has a new BCC crystal structure. This process helps the steel increase its elongation and impact resistance.
As the name implies, martensitic stainless steels are those that contain 11.5 to 18 percent chromium and less than 1% carbon. They are commonly used in cutting tools, springs, and tooling. One grade, called MA3M, contains molybdenum and is a low carbon/nitrogen alloy. These materials exhibit high corrosion resistance and are capable of achieving high hardness when hardened and tempered.
The microstructure of martensitic stainless steels is the key to their resistance to abrasion. In addition to high corrosion resistance, martensitic stainless steels are also tough and ferromagnetic. Their high toughness and strength make them ideal for use in a variety of applications. In addition to machining, martensitic stainless steels are used in cutlery, valve components, and special highly resistant wear.
The basic iron-chromium-carbon alloy is then added to increase its strength. Usually, an alloy of this kind contains 10.5% chromium. This amount is the minimum threshold for passivation. Some higher-carbon steels require assistance from a certified welder, while low-carbon martensitic steels are suitable for welding. In addition, precipitation-hardened stainless steels are welded, but the weldability of these materials depends on the grade used.
As the hardest structural component in steel, martensite is responsible for the hardness of most sharp edge tools. It has been known to smiths for over 3000 years and has played a critical role in civilization. The first martensitic stainless steels were developed by Brearly and Krupp Stahl in Sheffield, England, which brought fame and fortune to the city. They are still in use today.