Duplex stainless steel (DSS) is considered a perfect material and has many benefits as a main material of construction for applications in the industrial sectors. However, throughout the past several years the required thickness of DSS for application and product construction has grown substantially. This article will focus on how the growing thickness of DSS products causes the residual stresses within DSS to grow as well. In extreme sections the stresses can exceed tensile stress, which will lead to a crack. Just because a DSS product does not show any signs of cracking, does not mean that there are no existing residual stresses.
In order to combat this problem, it is important to use simulation software to calculate residual stresses. The moment the residual stresses are close to, or exceed yield stress, a fugitive emissions leak can occur from the moment the product is loaded, to the stresses at a level close to yield-stresses. The moment this subject is properly understood, a solution can be found for most cases. Residual stresses are not unique to just DSS and exist in all materials that need to be quenched during the production process of an application.
Detecting Residual Stresses
Residual stresses are stresses that have remained in a metal object, or in this case DSS applications, even after the original cause of the stresses has been removed. Any type of steel that requires rapid cooling/ quenching in water will be loaded with residual stresses. This can be explained quite easily: the moment a DSS product has been quenched in water, the outside surface, approximately 10 mm to 30 mm deep, will quickly become cold. The moment this outside layer becomes cold, it means it has shrunk and detecting residual stress in duplex stainless steel applications the outer section has been fixed. The inside volume must cool down and shrink as well. The cool-down process can only be done by convection. The shrinkage will cause compressive stresses on the outer, cold section of the product, and will therefore cause tensile stresses within the inside volume. The product will crack the moment the tensile stress exceeds the tensile properties of the steel.
Figure 1 shows a carbon steel section that has been quenched and tempered, and it helps visualize residual stresses in a steel section, which has been sawed for testing purposes.
It is clear that the inside of the carbon steel section is down after sawing occurred, while the outside section is up. As this was the first encounter personally witnessed, an understanding began to form of the level and effect of the residual stresses in steel products. It became evident that proper communication with the expert performing the sawing is crucial, as they will report extreme situations of residual stress that have been discovered.
Quenching DSS
Fast quenching of DSS is extremely important because it is crucial to minimize, and ultimately, avoid detrimental phases. It is important to maintain the ideal austenite ferrite ration of 50%:50% in the full thickness by quenching at the correct temperature for the grade of DSS. It is also crucial to remember that due to the thickness of the products, the speed of the cool-down process is different between the surface and the inside of the product.
The graph in Figure 4 shows the continuous cooling transformation (CCT) curve with three lines, from
50°C/minute at the outside, to 30°C/ minute at the inside of the product. The graph also shows the predicted level of detrimental phases to be expected in the product.
It is important to understand that the cool down speed is different in every quench-basin. Therefore, each
quench-basin is calibrated with a special test by logging multiple thermocouples on a thick block. The calibrated cooling speeds are used in the simulation software.
Heat Treatment
It is crucial to have an understanding and be familiar with the full heat treatment cycle. For stainless steels, the full cycle must include the quenching operation. The temperature curve of quenching is perhaps more important than understood, up until now.
Residual Stresses
The simulation software not only calculates the cooling speed but also the residual stresses inside the
product. Figure 5 shows the most important section of a product with the residual stresses of one side of a cylindrical hollow section of a forging. It shows the residual stresses in X, Y, and Z direction from left to right. The blue color represents compressive stresses in a range of 700 to 800 MPa in all directions. Green to orange represent tensile stresses, in which orange is in a range of 615 to 800 MPa in all directions.
The final products are machined in steps, with waiting and relaxation times in between. The reason for machining in steps becomes clear by studying the pictures. If the material is removed from the hollow part and the surfaces, DSS is being removed under compressive stresses. This then leads to dimensional changes.
The simulation in Figure 5 was the result after two improvement steps were made in the shape, and after
the product passed the final tests. The original shape and the first improvement shape were different,but both shapes were pressure-tested at 1.4 times the normal working pressure.
During the pressure testing, the inner stress level range was between 380 to 420 MPa. The pressure tests created relaxation/equalization of the stress-levels of the product, which then formed small surface shape deviations. The products perfectly passed the pressure test, but when they were pressure-tested again at a later stage by the final customer, the product was slightly leaking. As mentioned above, the design was improved in two steps to overcome this phenomenon. In addition to changing the final shape,
interrupted quenching was also performed during the last quenching process, which is a new feature
added to the quenching technology. By using the simulation technology, knowledge about residual stresses within the industry has immensely improved.