Introduction
Stainless steel is an important component in the modern industry having various uses in the industry. This has forced new developments in its manufacturing especially in terms of its composition. This can be the steel made from aluminum, manganese, and iron. All these materials makes the finished stainless steel product unique in terms of its composition, physical metallurgy, and other factors. This paper is going to discuss the development in stainless steel technology with regards to its composition in terms of the materials used. The materials that are going to be discusses are Al, Mn, and Fe. The paper is going to further discuss the various properties of the new developments in terms of the composition, physical metallurgy, mechanical properties, corrosion properties and the application of the said stainless steel material.
The discovery of stainless steel is generally believed to have begun in Europe and as such several people have laid claims to its discovery. The international stainless steel forum, recognizes Harry Brearley born in England as the inventor. This however, remains to be a controversial issue. However, the benefits that have been realized as a result of this discovery have contributed overwhelmingly to the overall well-being of humanity. Their application carries across a wide range of uses from low-end appliances such as utensils to highly sophisticated appliances such as space crafts. Research done in this subject area, has resulted in significant advancements
Stainless steels are a large family of specialised steels in which the first and the most important property is the corrosion resistance. Therefore, stainless steels are widely used in various industries where corrosion and oxidation resistance in operating environment is required. Stainless steels are also used widely for high temperature creep resisting or heat resisting applications.
Classification of stainless steels
Stainless steels can be classified into three major categories which are; Austenitic, Ferritic and Martensitic
Austenitic stainless steels contain 18-25% Cr with 8-20% Ni and low carbon (0.1% C maximum). The crystal structure of austenitic stainless steels is face-centred cubic (FCC). The structure of 18% Cr steel can be made fully austenitic by the addition of 8% Ni. Corrosion resistance of 18% Cr - 8% Ni is superior to that of the ferritic or martensitic steels. Austenitic Iron-Aluminum-Manganese alloys are possible substitutes for austenitic stainless Steels..
Ferritic stainless steels contain 15-30% Cr and low carbon, with some molybdenum, niobium or titanium. Nickel is not added in ferritic- stainless steel. The crystal structure of this type of stainless steel is body-centred cubic (BCC) and they remain ferritic over the whole solid state temperature range.
Martensitic stainless steels contain 12-17% Cr and 0.1 to 1.0% C. These steels are austenitic in the temperature range of 950 to 1000°C but transform to martensite on cooling.
This paper focuses on the recent developments in Fe-AL-Mn alloy stainless steel with special focus in the following:
Composition
Physical Metallurgy
Mechanical Property
Corrosion Property
Application
The composition
Generally alloying elements are added to steel to modify its mechanical properties due to the following reasons:
Improve hardenability
In the Fe-Al-Mn alloy the constituting elements contribute to the overall steel as below:
Figure 1: 3D-mechanism map for Fe-Mn-Al-C alloys.
Aluminum
The addition of aluminum to Fe-high Mn TWIP (transformation induced plasticity) steels has several functions. Aluminum increases Stacking Fault Energy (SFE) significantly and therefore stabilizes the austenite against the strain-induced transformation that occurs in the Fe-Mn alloys during deformation. Furthermore, it strengthens the austenite by solid solution hardening. Finally, owing to its high passivity, aluminum enhances the corrosion resistance of such steels.
Manganese
Many new steel concepts rely on Mn alloying. In medium Mn steels (MMnS) the Mn addition is between 5 and 12 mass% while in high Mn steels (HMnS) the Mn content is usually between 15 and 30 mass%. The steels develop either an austenitic or a ferritic/austenitic basic structure .
Figure 2: Variation of SFE as a function of Mn content in Fe-Mn alloys
Iron
Generally, the existence of iron in the alloy slightly increases the yield strength, but significantly reduces the elongation. The ultimate tensile strength maintains at similar levels when Fe contents is less than 0.5 wt%, but decreases significantly with the further increased Fe concentration in the alloys. The higher the iron concentrations in the alloy, the significantly more the elongation decreases. This is accompanied by a slight enhancement of the yield strength at increased iron level in the alloys. The ultimate tensile strength maintains at similar level when Fe is less than 0.6 wt%, but it decreases significantly when the Fe contents further increases.
They make the steel a high temporature material tha cen withsatnd a very high temperatuer and corrosion
Figure 3: Ferrite microstructure showing the grains
Physical Metallurgy
High strength alloys based on Fe-Mn-Al-C represent a new group of high manganese alloys with high aluminum content; the mostly contain 12.7 -25.6% Mn, up to 14.4% Aluminum. Stainless steel can have ferritic, austenitic or even multiphase structure depending on the content of primary alloying elements of C, Mn or Al which define the deformation mechanism as well.
The matrix phase of low-density steels based on a Fe-Al-Mn-C system can be either ferrite, austenite or a mixture of ferrite and austenite depending on the content of austenite stabilizing elements of C and Mn. Phase diagrams for Fe-5Mn-(0-9) Al are calculated based on the CALPHAD approach [23] and reproduced as a function of carbon at various temperatures from 500 to 1200 °C in figure 1. At 0% Al, as shown in figure 1(a), austenite has a single-phase region in a wide C range from 0 to 1.9% at high temperature, but exists along with ferrite and cementite (0) at lower temperatures. A eutectoid reaction for austenite decomposition to a lamellar ferrite and cementite microstructure occurs at approximately 0.6% C. A phase diagram with 3% Al in figure 1(b) shows a reduced austenite single-phase region; the increased Al content raises the stability of ferrite and suppresses the formation of an austenite single-phase region at low C content below 0.16%. However, with 3% Al, k -carbide is observed at low temperature on the high carbon side. k-carbide is present up to 650 °C at 2.0% C. Increasing the Al content to 6% resulted in a further decrease in the stability of austenite to higher temperatures and higher C content in figure 1(c). The higher Al content also increased the stability of k-carbide to higher temperatures and lower C content at the expense of cementite stability. A eutectoid reaction for austenite decomposition to lamellar ferrite and k-carbide microstructure occurs at approximately 1% C. As the Al content increased to 9%, the austenitic region is limited to higher C concentrations and higher temperatures while k-carbide stability increased and replaced cementite in the whole C range at low temperature as shown in figure 1(d) .
Corrosion Property
Several studies into this property have found that the steel is characterized by a partially recrystallized austenitic microstructure with numerous annealing twins and slip bands. According to the results of potential-dynamic analyses it was found that the samples of examined steel show poor corrosion resistance in the NaCl solution. The observed corrosion pits are related to the chemical composition. It is connected with the high dissolution rate of Mn and Fe atoms in NaCl solution. Fractographic analyses of samples revealed corrosion products on their surface in a form of pits with diversified size.
It has also been established that in the medium of HNO3 solution, the steel with high concentration of Al is covered by a thin layer of salt. After increasing Mn concentration to 30% and introducing additionally 6%Cr, the steel undergoes passivation and is covered by considerably more stable film of oxides. In addition, the steel containing Al and Cr shows resistance to the effects of 3.5% NaCl, that is not present in case of the steel containing only increased concentration of aluminum .
The observed corrosion products are related to the chemical composition rather than to the phase structure of the steel. It is connected with the high dissolution rate of manganese and iron atoms in NaCl solution.
The various types of corrosion that stainless steel product will undergo through can be classified as contact galvanic corrosion which will occur due to the presence of two metals in the electrolyte leading to the creation of a difference electrical potential. The corrosion will lead to the attack of the anode and the further corrosion of the metallic component. This is solved by the use of the aluminum in the steel. Aluminum will form a layer of Aluminum oxide that will prevent oxidation of the anode and further corrosion of the terminal .
The pitting corrosion shown in the figure above is as a result of exposure of steel to NaCl solution with a dopant of hydrogen peroxide. Some pits in the steel will cause penetration and the right hand image will be used to show the magnification of the pitting corrosion. The proneness to this type of corrosion will be magnified by the increased concentration of chloride, oxidants, temperature and a decreased ph level
The figure on cracking corrosion is for a stainless steel tank used for the storage of hot water. The various classes of the steel will have different properties at different temperatures and this determines the cracking in the materials.
The figure shows the corrosion caused by formation of chromium carbide in the grains of the steel. The heating of the steel at some temperatures will lead to the binding of the carbon in the media and thus lead to the sensitization of several zones that causes weakening alongside them .
Mechanical Property
There are several materials that make up stainless steel and the three named materials, Fe, Mn and Al will make it have different mechanical properties. The mechanical properties of the steel varies with the degree of usage assigned to it and the working can either strengthen it more or less. Materials like austenitic steel can only be hardened by cold working while others can only be strengthened by heat treatment. This determines the strengthening of the mechanical properties of the materials and the subsequent weakening of the mechanical properties of the steel
The mechanical properties of the steel will be determined by the size of the materials under test, the material composition of the steel alloy and the working condition of the materials. This can be shown in the table below where the individual property of the steel is analyzed in nuts and bolts. This can be in terms of tensile strength, stress and strain, and the elongation after fracture that the material will undergo. The property class of the steel will be of great importance in the determination of the mechanical properties of the steel. Class 50 materials will be used as fasteners and will be strengthened through the turning and the hot pressing of the materials. The class 70 is for the cold formed fasteners. The class 80 will be the highest mechanical property class with an improved mechanical value deformation. This three properties are the classes of the grades of the steel like A1, A2, and A3 which is determined from different manufacturing
Applications:
The summary of applications of the stainless steel is outlined below :
It is used in the manufacture of cutlery and other cookware where it gives a great experience and the recommended alloy should not have more than 16% chrome because of the corrosion that can arise with the use of dishwashers.
It is used also in the manufacture of dishwashers and other washing machines. This has a downsied of being prone to crevice corrossion.
Stainless steelis used in the building architecture and construction. This can be outdoor and indoor where it gives very positive peroformance criteria. For industrial environment and use at the outdoor environment near a coastal region, it stainless steel that has a specified percentage of Nickel is not encouraged
In the use in the food and beverage industry, stainless steel perfoms better in the storage of the foods that have a pH higher than 3 . There are some restrictions in its use especially with a higher percentage of nickel
In the transportation industry, stainless steel is used in the manufacturing of the body of a bus and it gives a very good performance. It is also used in the decoration of motorcycle rims and can also be used in the manufacturing of the chemical tanks although it is highly discouraged.
It is used in the manufacture of chemical tanks
Analysis of Steel
The mechanical properties are greatly influenced by stacking fault energy as far as fully austenitic alloys with high Mn (> 15%Mn) and low Al (< 3-6%Al) contents are concerned. However, when the Al content is higher, mechanical properties are also controlled by k -carbide precipitation .
According to the results of potentiodynamic analyses it was found that the samples of examined steel show poor corrosion resistance in the NaCl solution. The observed corrosion products are related to the chemical composition rather than to the phase structure of the steel. It is connected with the high dissolution rate of manganese and iron atoms in NaCl solution. The low stability of Mn leads to forming unstable layer and preferential dissolving of manganese at the oxide/electrolyte interface
Mn is the main element retarding the rate of static recrystallization, while Al and Cr contribute in a minor way only. In the case of compositions, which induce ferrite formation at high temperatures, static recrystallization is very rapid. A simple regression model can be used to predict the static recrystallization rate in stainless steels under given high temperature deformation conditions .
The tensile properties of stainless steel steels depend essentially on the stacking fault energy, which is related to Mn and Al contents and test temperature. At a constant Mn content, Al increases the stacking fault energy, and therefore the deformation mode can change from strain-induced martensitic transformation to deformation twinning and finally to dislocation glide. Consequently, strain hardening rate, elongation and tensile strength vary in quite a complicated way .
The corrosion resistance of Fe-high Mn-Al steels is not dependent on their phase structure, but rather on the chemical composition
Conclusion
Steels has a variety of chemical composition and it is dependent on its intended use. The various components that areused in the design and manufacture of modern day steel are manganese, iron and aluminium. They have various properties that make the steel made form them to be used in avarious applications.The manganese in steel is mainly for the promotion of stability at near room temperature and will be the property that will increase hot working conditions. A lower percentage will not have any great effect on the ductility of the steel, strength and toughness. Aluminium will be used to lower the hardenability of steel. It also serves to improve. scaling. Ferrite in the steel will be mainly in applications where the key thing is the resistance to corrosion
The physical mertullugy, mechanical properties and the corrosion effects on this materials is dependant on different factors and this can be shown by the various tests conducted on these materials. Stainless steel finds numerous applications in the modern world be it from the normal day-day operation to the complex application that requires a better alloy of the steel. The strain stress properties of these materials is different based on the different composition of the materials.
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