History of Stainless Steels
Stainless steels are used from the simplest materials we use in our daily lives to more complex designs. Stainless steel, the most important class of alloys, was developed in Great Britain and Germany in the early twentieth century. Although there are non-corrosive-resistant non-ferrous metals (such as Nickel, Nickel-Silver (Nickel-Copper-Titanium alloys that do not contain silver but are similar), Copper, Brass, and Bronze) available, these materials are not as cost-effective and durable as stainless steels.
Toward the late 1800s, despite the use of chromium-containing steels, it was not known that chromium formed a corrosion-resistant oxide layer on the surface. In 1913, while developing rifle barrel steels, Harry Brearley discovered martensitic stainless steels. Later, he observed that a steel containing 0.3% carbon and 13% chromium was resistant to wear and corrosion. In 1915, Brearley announced the discovery of stainless steel, and the article published in The New York Times is presented in Figure 3. 420-grade stainless steel laid the foundation for the cutlery production industry and is still in use today.
During the same period, in Germany, steel with a high nickel content, while resistant to tarnishing, was not sufficiently resistant to corrosion. In 1912, Benno Strauss and Eduard Maurer discovered chrome-nickel-based austenitic stainless steel. Austenitic stainless steels quickly replaced martensitic and ferritic stainless steels. This steel was soon used in the production of nitric acid storage tanks for the chemical industry. In America and France, various exploratory discoveries were made independently of these developments.
Figure: Harry Brearley and Bearley’s article in the New York Times.
In the 1920s and 1930s, rapid developments were observed in stainless steel grades that still have a wide range of applications today, such as AISI 302 (18% Cr, 8% Ni), AISI 316 (18% Cr, 8% Ni, 2.5% Mo), AISI 410 (12% Cr), and AISI 430 (16-18% Cr). In the 1950s, stainless steels were considered precious metals and priced accordingly. The nickel crisis that emerged worldwide in the 1970s, along with the increase in nickel prices, led to new developments in stainless steel production. As a result of the studies conducted during these years, it was discovered that nitrogen, when dissolved as an interstitial element in the stainless steel structure, is a strong austenite former. Even in small quantities, it was determined that the use of nitrogen allowed for a reduction in the nickel content in the chemical composition of stainless steel. As a result, during these years, the production and usage of interstitially reinforced stainless steels became widespread.
1.1 Alloying Elements and Metallurgical Effects of Stainless Steels
Each of the alloying elements has a specific effect on the properties of steel. It is the combined effect of all alloying elements and to some extent impurities that determine the property profile of a particular grade of steel. To understand why different grades have different compositions, it is necessary to know the alloying elements and their influence on structure and properties. It should also be noted that the effect of alloying elements differs in some aspects between hardenable and non-hardenable stainless steels.
I. Carbon (C)
It is found in the material in varying proportions ranging from 0.025% to 0.040% and is one of the fundamental alloying elements in the stainless steel structure. The reason for these low ratios is to prevent chromium carbide precipitation and to provide stainless properties to the steel. At the same time, the carbon content has a great influence on weldability. It can especially lead to the depletion of chromium and, consequently, a decrease in corrosion resistance in areas subjected to heat effects. With increasing carbon content, this risk can lead to hardening and embrittlement of the material.
Carbon is a strong austenite former and supports a strong austenitic structure. It also significantly increases mechanical strength. Carbon reduces resistance to intergranular corrosion. Under heat treatment or in welding, carbide formation at the grain boundaries may cause further intergranular opening. In ferritic stainless steels, carbon significantly reduces both toughness and corrosion resistance.
In martensitic and martensitic-austenitic steels, carbon increases hardness and strength. The increase in hardness and strength in martensitic steels is usually accompanied by a decrease in toughness, and in this way carbon reduces the toughness of stainless steels.
II. Chromium (Cr)
Chromium is the most important alloying element in stainless steels. It is the key alloying element that provides corrosion resistance to stainless steels through the formation of a chromium-rich (Fe,Cr)2O3 layer on the material’s surface. It is the primary alloying element that imparts basic corrosion resistance to stainless steels. Corrosion resistance and resistance to scaling increase with higher chromium content. Additionally, it enhances resistance to oxidation at elevated temperatures. Chromium supports a ferritic structure; it is a ferrite former.
III. Nitrogen (N)
Nitrogen is a very strong austenite former and supports a robust austenitic structure. It also significantly enhances mechanical strength. Nitrogen, especially in combination with molybdenum, increases resistance to localized corrosion. It is used in high-chromium, low-carbon steels to prevent grain growth at high temperatures. However, at low temperatures, it reduces the weld metal toughness.
In ferritic stainless steels, nitrogen will significantly reduce toughness and corrosion resistance. In martensitic and martensitic-austenitic steels, nitrogen increases both hardness and strength but reduces toughness.
IV. Nickel (Ni)
The primary reason for the addition of nickel is to promote an austenitic structure. Nickel generally enhances ductility and toughness. It also reduces the corrosion rate and improves homogeneous corrosion resistance. Therefore, it is advantageous in acidic environments. In precipitation-hardening steels, nickel is used to form intermetallic compounds that increase strength. It has a beneficial effect on weld metal toughness. Hence, stainless steel welds are often made using filler wires containing nickel.
V. Copper (Cu)
Copper increases corrosion resistance in specific acids and supports an austenitic structure. In precipitation-hardening steels, copper is also used to form intermetallic compounds that increase strength.
VI. Manganese (Mn)
Manganese is commonly used in stainless steels to improve hot ductility. Its effect on the ferrite/austenite balance varies with temperature: at low temperatures, manganese acts as an austenite stabilizer, while at high temperatures, it stabilizes the ferrite phase. Manganese increases the solubility of nitrogen and is used to achieve high nitrogen content in austenitic steels.
VII. Titanium (Ti)
Titanium is a strong ferrite former and carbide former, effectively reducing the effective carbon content and promoting a ferritic structure in two ways. It is added to austenitic steels to enhance resistance to intergranular corrosion and improve mechanical properties at high temperatures. In austenitic stainless steels, it is used as a balancing element to prevent chromium carbide precipitation. Titanium has a higher affinity for carbon, so it forms compounds with carbon before chromium does within the structure. This prevents the formation of free chromium, ensuring the formation of a passive layer after welding and heat treatment.
In ferritic stainless steels, titanium is added to improve toughness and corrosion resistance by reducing the amount of intermediates in solid solution.
In martensitic steels, titanium reduces martensite hardness and enhances tempering resistance. In precipitation hardening steels, titanium is employed to form intermetallic compounds for increasing strength. It is added with aluminum (Al) to influence aging hardening in high-strength and heat-resistant alloys.
VIII. Niobium (Nb)
Niobium is both a strong ferrite and carbide former. Similar to titanium, it is a moderate ferrite former. In austenitic steels, it is added to enhance resistance to intergranular corrosion, used to balance against chromium carbide precipitation, and also improves mechanical properties at high temperatures.
In martensitic steels, niobium reduces hardness and enhances tempering resistance. It is added to certain martensitic stainless steel types with the aim of binding carbon to reduce the steel’s hardening tendency. It is also introduced into some high-strength alloys to influence both hardness and strength. In the United States, it is also referred to as Columbium (Cb).
IX. Silicon (Si)
Silicon increases resistance to oxidation in both high-temperature and low-temperature strong oxidizing solutions. It is a moderate ferrite former. In quenched and tempered steels, it elevates tensile strength and yield strength but reduces cold-forming ability. It also enhances the electrical resistance of steels.
X. Molybdenum (Mo)
Molybdenum significantly enhances resistance to both general and localized corrosion. It is used to increase general corrosion resistance in non-oxidizing environments and pitting corrosion resistance in other environments. It slightly increases mechanical strength and strongly supports a ferritic structure. Molybdenum also promotes the formation of secondary phases in ferritic, ferritic-austenitic, and austenitic steels. In martensitic steels, it will increase hardness at higher tempering temperatures due to its effect on carbide precipitation. It enhances strength and creep resistance at high temperatures.
XI. Aluminum (Al)
It is a strong ferrite former. When added in significant quantities, aluminum enhances oxidation resistance. For this purpose, it is used in some heat-resistant alloys. In precipitation hardening steels, aluminum is used to create intermetallic compounds that increase strength in the aged condition. When added to certain high-strength alloys along with titanium, it reduces the effect of aging hardening. When added to welding metal containing 12% chromium, it transforms the structure into ferritic, making it non-hardenable.
XII. Cobalt (Co)
Cobalt is used solely as an alloying element in martensitic steels, where it particularly enhances hardness and tempering resistance, especially at high temperatures. It is also added to many stainless alloys to improve their high-temperature creep and strength properties.
XIII. Vanadium (V)
Vanadium increases the hardness of martensitic steels due to its effect on the existing carbide type. It also enhances tempering resistance. Vanadium stabilizes ferrite and promotes ferrite in high-content structures. It is only used in hardenable stainless steels.
XIV. Sulfur (S)
To enhance machinability, sulfur is added to some stainless steels, increasing their susceptibility to chip production. Excessive amounts of sulfur will significantly reduce production characteristics such as corrosion resistance, ductility, and manufacturability, including weldability and formability.
XV. Cerium (Ce)
Cerium is one of the rare earth metals, and it is added in small quantities to some heat-resistant temperature steels and alloys to enhance resistance against oxidation and high-temperature corrosion.
XVI. Tungsten (W)
Tungsten is a strong ferrite former. It is added to certain high-temperature alloys to increase strength and creep resistance.
XVII. Phosphorus and Selenium (P ve Se)
One of these elements, in small quantities, is added to stainless steel with either molybdenum or zirconium to improve its machinability. All three of these elements promote cracking in the weld metal.
Source:
Stainless Steel Flat Product Basics
Trinox Metal Technical Publications
ISBN: 978-625-00-0457-9
Nickel and Chromium Equivalencies
The other alloying elements have an effect on the formation of delta ferrite at the solution treatment temperature depending on whether they are elements that form austenite or ferrite. The impact of alloying elements on phase stability within stainless steel has been evaluated using nickel (Equation 1) and chromium (Equation 2) equivalent compositions, and these have been plotted on the Schaeffler diagram. This diagram allows for the determination of microstructure based on the ratios of alloying elements. Alloying elements that contribute to the creation of a Creep value stabilize the ferrite phase, and an increase in this ratio leads to an increase in the ferrite content, whereas alloying elements contributing to the creation of a Nie value stabilize the austenite phase, and an increase in this value results in an increase in the austenite phase.
Nieş = % Ni + % Co + 0,5 % Mn + 30 % C + 0,3 % Cu + 25 % N (1)
Creş= % Cr + 2 % Si + 1,5 % Mo + 5 % V + 5,5 % Al + 1,75 % Nb + 1,5 % Ti + 0,75 % W (2)
Figure: Schaeffler diyagram.
In this way, it is possible to consider the combined effect of alloying elements. The Schaeffler diagram was originally developed for weld metal, explaining the structure after melting and rapid cooling. However, it has been found that the diagram also provides a useful representation of the effect of alloying elements on processed and heat-treated materials. Nevertheless, in practice, processed or heat-treated steels with ferrite content in the range of 0-5% contain less ferrite than predicted by the diagram.
It should also be noted that the Schaeffler-Delong Diagram is not the only diagram available for evaluating the ferrite content and structure of stainless steels. Several different diagrams have been published, each with slightly different equivalents, phase boundaries, or overall arrangements. The influence of some alloying elements has also been the subject of significant debate. For example, the austenite-stabilizing effect of manganese has been thought to be smaller than originally predicted in the Schaeffler Diagram. Its effect is also temperature-dependent.
1.1 Precipitate Phases in Stainless Steel
Intermetallic Phases
Alloys containing transition elements such as Fe, Ni, Mn, Co, etc. (A), with transition elements of the type Cr, Ti, V, etc. (B), can form intermetallic phases with a formula ranging from A4B to AB4. When exposed to high temperature, austenitic stainless steels are known to precipitate a number of secondary phases. Some of these phases have large effects on mechanical and corrosion properties.
Sigma, chi, carbides, R-phase, Laves phases, G-phase, mu phase, Z-phase, etc., are some of the phases that can precipitate in austenitic stainless steels.
Sigma Phase (σ)
The sigma phase has a Tetragonal Body-Centered (TBC) crystal structure. The values of Cr and Ni equivalents can be used to assess the likelihood of σ-phase formation in a Fe-Cr-Ni alloy at high temperatures. The precipitation tendency of the sigma phase in austenitic stainless steels depends on the chemical composition of austenite after the precipitation of the first-formed carbides and nitrides.
Figure: A section of the Fe-Cr-Ni ternary phase diagram at 650°C.
The formation tendency of the σ-phase in an austenitic stainless steel can be explained by the equation proposed by Hull. The formula for Equivalent Chromium Content (ECC) is as follows,
ECC =% Cr + 0.31 % Mn + 1.76 % Mo + 0.97 % W + 2.02 % V + 1.58 % Si + 2.44 % Ti + 1.7 % Nb + 1.22 % Ta – 0.266 % Ni – 0.177 % Co
If the Equivalent Cr Content (ECC) is greater than 17-18% by weight, the steel is sensitive to sigma phase formation. Some studies have shown that as the carbon content in the structure decreases, the precipitation of the σ-phase becomes easier.
NECC = ECC / % C
At 700 °C, the effect of various alloying elements on the kinetics of σ-phase precipitation is given below. Chromium, molybdenum, titanium and niobium all favor σ formation, while the precipitation rate increases with the addition of 2 to 3% silicon. The inclusion of nitrogen in the weld deposit prevents or delays the nucleation of the σ and χ phases. Delta-ferrite and grain boundaries affect the growth kinetics of sigma and other intermetallic phases, but not the total content of these phases.
Figure: Sigma phase precipitation rate at 700°C.
Cold treatment shortens the precipitation time for the σ-phase formation. Conversely, an increase in grain size due to high-temperature annealing delays σ-phase precipitation. The presence of ƍ-ferrite, especially in welds, can shorten the incubation period for σ-formation in austenitic stainless steel. Stress accelerates σ-phase precipitation and widens the formation range to lower temperatures.
The precipitation of the σ-phase is controlled by both the diffusion rate of chromium and other σ-forming elements, as well as the nucleation type. The chemical composition of the σ-phase has been studied for different types of austenitic steels (17 Cr-12 Ni-2.5 Mo-Ti, 25 Cr-20 Ni-0.03 & 0.13 C-0.6 & 2 Si) with heat treatment at various temperatures and durations ranging from 10 to 5000 hours. It has been found to vary with time and temperature, particularly within the temperature range of 650 to 900°C. The compositions of ƍ-ferrite and σ-sigma phases are close to each other. Therefore, in the weld metal of austenitic stainless steel, ƍ-ferrite can easily transform into the σ-phase through a crystallographic transformation. Heat input during welding plays a significant role in the precipitation kinetics of σ and other intermetallic phases. Higher heat input into the weld metal delays the decomposition kinetics of ƍ-ferrite and, consequently, the precipitation kinetics of the σ-phase. It has been reported that vermicular σ-phase exhibits a faster precipitation kinetics in comparison to dendritic ferrite. It is known that the sigma phase affects the tensile and toughness properties of stainless steel.
Chi Phase (ꭕ)
The χ-phase has a body-centered cubic structure and is a stable intermetallic compound containing Fe, Cr, and Mo. Chi is a carbon-dissolving compound of the M18C type. The composition of χ can vary significantly with changes in alloying elements.
Figure: Sigma (σ) and Chi (χ) phases.
Ferritic-Austenitic (Duplex) Stainless Steels
Duplex stainless steels are a type of high-alloy steel that contains both ferritic and austenitic phases, making them a material with high corrosion resistance and excellent mechanical properties. Due to their superior characteristics compared to single-phase stainless steels, they are preferred in various fields today. Despite being more expensive than other grades, they can be economically evaluated in terms of performance-cost ratio.
In varieties of these steels referred to as Super Duplex, the ideal phase ratio of austenite to ferrite is %50/%50. The ferritic phase provides resistance to mechanical and stress corrosion cracking, while the austenitic phase imparts ductility and general corrosion resistance. Additionally, the presence of the ferritic phase enhances pitting corrosion resistance. The most commonly used duplex stainless steel is AISI 2205. The ferritic phase is body-centered cubic, whereas the austenitic phase is face-centered cubic.
They exhibit better stress corrosion resistance compared to austenitic steels and better toughness and ductility compared to ferritic steels. Additionally, when both phases are present, even in the annealed condition, they demonstrate yield strength between 550 and 690 MPa, which is approximately twice the yield strength of single-phase steels of the same type. The current commercial grades contain approximately 22-26% chromium, 4-7% nickel, a maximum of 4.5% molybdenum, about 0.7% copper, and 0.08-0.35% nitrogen.
The classification of duplex stainless steels is done using the PREN (Pitting Resistance Equivalent) equation. The resistance of the material to pitting corrosion is directly proportional to the value obtained from the PRE equation.
PREN=%Cr + 3.3%Mo + 16%N (7)
- 23% Cr-containing, Mo-free (PREN≈25)
- 22% Cr and MO containing (PREN≈30-36%)
- 22% Cr and MO containing (PREN≈30-36%)
- Super duplex stainless steels (PREN>40)
Main Properties of Ferritic-Austenitic (Duplex) Stainless Steels
- They provide high resistance against stress corrosion.
- They show higher corrosive resistance in environments where chlorine ion is not present.
- They provide higher mechanical strength than austenitic and ferritic steels.
- They have good formability.
- They have high weldability.
Uses of Ferritic-Austenitic (Duplex) Stainless Steels
Despite the high qualities such as ductility, toughness, and corrosion resistance offered by duplex stainless steels, the mechanical strength and pitting corrosion resistance can be compromised due to the secondary phases formed as a result of high heat input. This limitation restricts the applications and operating temperatures of duplex stainless steels.
While this grade of stainless steel may have a higher cost per kilogram compared to other grades, it stands out in terms of price-performance due to its ability to achieve high levels of durability even in thinner materials. Widely utilized in areas requiring robustness, flexibility, and durability, this quality finds applications in pump shafts, boat propellers, specialized hydroelectric plants, the food industry, the chemical sector, and machinery production. Duplex stainless steel offers a high degree of mechanical strength and corrosion resistance, in addition to being as flexible as austenitic stainless steels. For these reasons, it is recommended to use this grade of stainless steel in any application where quality, lightness, and robustness are sought after.
Table: Chemical Properties of some Duplex Stainless Steels
| Alloy | UNS No | EN No | C (%) | Cr (%) | Ni (%) | Mo (%) | N (%) | Others (%) |
| 2304 | S32304 | 1.4362 | 0.030 | 21.5-24.5 | 3.0-5.5 | 0.1-0.6 | 0.05-0.6 | Cu:0.01-0.6 |
| 2205 | S31803 | 1.4462 | 0.030 | 21.0-23.0 | 4.5-6.5 | 2.5-3.5 | 0.08-0.20 | |
| 255 | S32550 | 1.4507 | 0.04 | 24.0-27.0 | 4.5-6.5 | 2.9-3.9 | 0.10-0.25 | Cu:1.5-2.5 |
| 2507 | S32750 | 1.4410 | 0.030 | 24.0-26.0 | 6.0-8.0 | 3.0-5.0 | 0.24-0.32 | |
| Z100 | S32760 | 1.4501 | 0.030 | 24.0-26.0 | 6.0-8.0 | 3.0-4.0 | 0.20-0.30 | Cu:0.5-1.0
W:0.5-0.1 |
Eriksen Test
This test is conducted on finished flat products, including all manufacturing processes, such as annealing and skin-pass. Typically applied to steels with a ferritic matrix, this test can provide insights into deep-drawing suitability and surface quality for stainless steel flat products. The cup test is generally similar to a deep-drawing process, making it an essential test for flat stainless steel producers. In particular, the consequences of improper heat treatment and tempering can be clearly observed in this test.
The points to be considered in sampling are as follows;
The sample should represent the whole without any processing.
A strip of approximately 90-110 mm in length (may vary depending on the device) is cut from the sample to fit properly into the relevant test area of the apparatus.
BThis sampling process is performed by taking a semi-finished product from at least one location that will represent the coil. If the sample is to be taken from the beginning or end, it should be cut sufficiently inside (30-40 mm) from the ends of the sheet. This eliminates the ‘over-thick’ portion that exceeds thickness tolerances due to the rolling process and ensures the removal of surface defects that are likely to be present at the beginning and end of the sheet.
Test Implementation
The sample obtained by cutting as shown in the figure is tested in the device.
ŞFigure: Sample Specimen.
- The sample is placed in the chamber and the cover of the device is put in place by means of the handle.
Figure: Eriksen Test Apparatus.
- After the sample is connected as shown in the image, the pressing process starts.
Figure: Eriksen Test Process.
- After sufficient depth is reached, the sample is removed and examined.
- A coaster-like appearance is obtained on the sample and the sample is worked on at certain intervals from right to left, from decreasing depth to increasing depth.
- As a result, the incorrect (inappropriate) appearance that we do not want to see is the vascularization that occurs as in the picture below.
Figure: Appropriate Test Result (a), Inappropriate Test Result (b).
- If there is no non-conformity, the finished/semi-finished product is approved and sent to the next process. If an error like the one mentioned above occurs, the non-conformity is documented. In this case, the material will either tear during the deep-drawing process or exhibit poor surface characteristics..
In this test, we gained insights into the effect of the appropriate heat treatment and tempering processes on the surface quality and deep drawing capability in the deep drawing process. If the material’s elongation capacity is low, we would observe cracks and tears at the bending points and especially at the areas in contact with the mold. In particular, in ferritic stainless steels, inadequate heat treatment and tempering conditions may lead to the appearance of Lüders bands.
To conduct a detailed examination, macroscopic analysis can be performed on the samples subjected to the cup test. These samples can be retested, especially for stress corrosion cracking and intergranular corrosion development. It is beneficial to examine the bending points of the samples at different magnifications using a hand microscope.
1.1 Welding Metallurgy of Stainless Steels
Austenitic stainless steels are generally readily weldable as no embrittlement structures form in the heat affected zone (ITAB or HAZ). However, a number of detrimental effects can occur.
These are:
- In a fully austenitic welding metal, the formation of hot cracking during solidification can be mitigated by introducing some ƍ-ferrite. This ratio is commonly mentioned in the literature as having around 15% ƍ-ferrite.
- In both the welding metal and the heat-affected zone (HAZ), various liquation cracks, especially in low melting point phases, can occur. Cracking issues in fully austenitic welds can be prevented by reducing the volume of the molten pool and decreasing the heat input. Naturally, these parameters can be adjusted within the limits and standards of metallurgical processes. Minimizing impurities and contaminants in the material and selecting electrodes that result in 5-10% δ-ferrite in the weld pool are the primary means of mitigating physical welding defects.
- Stabilized steels, especially those containing niobium, can dissolve some MC carbides due to the high temperature in the ITAB (intercritical temperature annealing) zone. These elements, when dissolved in the main structure, can precipitate in regions of high stress in the weld seam or ITAB when stress relief is performed. This can lead to creep fracture. The solution to this issue involves conducting a heat treatment above the dissolution temperature of the formed metal carbide. The ideal solution is to perform a solution annealing on the entire part and then reintroduce the desired properties through the necessary heat treatment.
- During welding, in the heat-affected zone, the heat input can lead to the formation of chromium carbide compounds at grain boundaries or in dislocation regions, typically as Cr23C6. In such a scenario, the free chromium content in the structure decreases. This results in selective corrosion in the region where chromium is depleted, known as weld decay. As a solution, a full solution treatment at 1050°C can be applied to dissolve carbides at grain boundaries in the heat-affected zone, or a tempering process at around 900°C (a ‘recovery’ process) can be used to allow the redistribution of chromium from the bulk to the depleted region. Austenitic stainless steels stabilized with Ti or Nb are susceptible to knife-line attack.
The probability of delta-ferrite formation during the cooling of the weld metal and the primary solidification mode can be understood from the 70% iron isopleth. If the chemical composition is known, the amount of retained delta-ferrite in the weld metal can be determined from phase diagrams. The Schaeffler diagram is one of the most popular phase diagrams used for this purpose. It positions important stainless steels on the diagram based on their tendency to form delta-ferrite. However, the Schaeffler diagram does not account for the influence of nitrogen, which is a potent austenite stabilizer.
The amount of retained delta-ferrite in the weld metal is determined based on service requirements. Generally, delta-ferrite tends to embrittle a weld metal and can compromise its corrosion resistance properties. However, it has been recently understood that the embrittlement of austenitic stainless steel weld metal is not solely due to the presence of delta-ferrite but is instead attributed to the cold working in the weld metal.
