The effect of copper alloying element on mechanical properties of stainless steels

"Stainless" steel is actually a generic term referring to a variety of steel types. Like all other kinds of steel, stainless steel is made primarily from iron and carbon in a two-step process. What makes stainless steel different is the addition of chromium (Cr) and other alloying elements such as nickel (Ni) to create a corrosion-resistant product. Steel corrodes because iron, the metal used to make steel, occurs in nature in combination with other elements. When iron ore is artificially manipulated into a pure form to make steel, it becomes unstable and will readily recombine with oxygen. When chromium is added to steel, it forms chromium oxide, which acts as a protective surface to prevent air and moisture from causing rust, as happens with ordinary steel. Chromium is added in quantities ranging from 10.5 to 30%, depending on the application or environment in which the steel is to be used. Stainless steel is usually divided into 5 types: Ferritic – These steels are based on Chromium with small amounts of Carbon usually less than 0.10%. These steels have a similar microstructure to carbon and low alloy steels. They are usually limited in use to relatively thin sections due to lack of toughness in welds. However, where welding is not required they offer a wide range of applications. They cannot be hardened by heat treatment. High Chromium steels with additions of Molybdenum can be used in quite aggressive conditions such as sea water. Ferritic steels are also chosen for their resistance to stress corrosion cracking. They are not as formable as austenitic stainless steels. They are magnetic. Austenitic - These steels are the most common. Their microstructure is derived from the addition of Nickel, Manganese and Nitrogen. It is the same structure as occurs in ordinary steels at much higher temperatures. This structure gives these steels their characteristic combination of weldability and formability. Corrosion resistance can be enhanced by adding Chromium, Molybdenum and Nitrogen. They cannot be hardened by heat treatment but have the useful property of being able to be work hardened to high strength levels whilst retaining a useful level of ductility and toughness. Standard austenitic steels are vulnerable to stress corrosion cracking. Higher nickel austenitic steels have increased resistance to stress corrosion cracking. They are nominally non-magnetic but usually exhibit some magnetic response depending on the composition and the work hardening of the steel. Martensitic - These steels are similar to ferritic steels in being based on Chromium but have higher Carbon levels up as high as 1%. This allows them to be hardened and tempered much like carbon and low-alloy steels. They are used where high strength and moderate corrosion resistance is required. They are more common in long products than in sheet and plate form. They have generally low weldability and formability. They are magnetic. Duplex - These steels have a microstructure which is approximately 50% ferritic and 50% austenitic. This gives them a higher strength than either ferritic or austenitic steels. They are resistant to stress corrosion cracking. So called "lean duplex" steels are formulated to have comparable corrosion resistance to standard austenitic steels but with enhanced strength and resistance to stress corrosion cracking. "Superduplex" steels have enhanced strength and resistance to all forms of corrosion compared to standard austenitic steels. They are weldable but need care in selection of welding consumables and heat input. They have moderate formability. They are magnetic but not so much as the ferritic, martensitic and PH grades due to the 50% austenitic phase. Precipitation hardening (PH) - These steels can develop very high strength by adding elements such as Copper, Niobium and Aluminium to the steel. With a suitable "aging" heat treatment, very fine particles form in the matrix of the steel which imparts strength. These steels can be machined to quite intricate shapes requiring good tolerances before the final aging treatment as there is minimal distortion from the final treatment. This is in contrast to conventional hardening and tempering in martensitic steels where distortion is more of a problem. Corrosion resistance is comparable to standard austenitic steels like 1.4301 (304). Stainless steels are the most interacting type of steel, especially when compared to other steel types. Today, stainless steel is widely used in almost all areas, from white goods to kitchen appliances, from the automotive sector to the health and food sector. Consumption of stainless steel per capita defines as an indication of the development between countries and includes in the product group, which is increasing consumption each year in our country. The rapid increase in the consumption of stainless steels has increased in expectation from the market in order to gain competitive advantage in the market. Increased demands on stainless steels where the stainless steel forms especially into more complex structures and needs high forming performance. In standart applications, the increase in nickel element content in the structure is expected to increase the performance of forming, but the nickel element has a higher unit price and it has been questioned the possibility of using another alloying element as an alternative. In this study, an alternative and a relatively cheap copper alloy element is used instead of the nickel alloy element to improves the forming performance and the mechanical properties. Two different sample groups are formed for the test. One of the samples is austenitic stainless steel, the other being austenitic stainless steel containing approximately 2% by weight copper. The specimens tested under similar parameters to investigate the change in mechanical properties and metallographic properties of the copper added alloy element through the production steps such as rolling, annealing and finishing. As a result of the tests, the yield strength and tensile strength of the austenitic stainless steels subjected to copper addition is decreased while the elongation value increased and the hardness value decreased. The results of the tests conducted by the deep drawing test indicate that the ability of forming the austenitic stainless steels with copper addition increased. No significant differences observed in the grain size, structure and grain distribution during metallographic tests. As a result, the addition of copper alloy element in austenitic stainless steel materials increased the ductility and the ability to form sheet materials, but it caused a weakness in corrosion properties. Corrosion properties have not been fully explored yet, are excluded from this study, and are intended to be examined in detail in future studies.