Analyzed in terms of composition, stainless steel is an alloy composed of a large amount of chromium content, usually no less than 11% by mass. Chromium is responsible for corrosion resistance, so any increase in chromium will make the metal more resistant to corrosion. It’s aesthetically pleasing, easy to produce, clean, and maintain, and environmentally friendly, making it a top choice for components for architecture, automobiles, and many other products.
In precision casting, stainless steel casting is the most cost-efficient solution for metal components used in food machinery, medical equipment, the oil field industry, and other applications.
Why stainless steel casting?
Casting stainless steel is a great method for creating intricate stainless steel components. Precision stainless steel parts can be produced for a variety of industries, including the automotive, food machinery, oil and gas, medical, etc., using the investment casting technique. So why cast stainless steel?
Investment casting in stainless steel may reduce the labor of machining and only need to leave a small machining allowance in the tough areas since it has excellent dimensional accuracy and surface polish. Some stainless steel castings, on the other hand, have a net form structure and can be used directly. Because of this, using the stainless steel casting method can significantly reduce the need for mechanical processing equipment and processing time, as well as raw materials.
The ability to cast a range of different grades of complicated parts, including duplex stainless steel, is another benefit of the stainless steel casting technique. For instance, mechanical machining is difficult to use to manufacture stainless steel exhaust manifolds. In addition to batch manufacturing, which ensures the consistency of the casting, using the stainless steel casting technique can also prevent the stress concentration of leftover cutting patterns after machining.
Difference between different cast stainless steel grades
Many different types of stainless steel are suitable for casting, each with its unique properties and characteristics. Some of the most common types of stainless steel include:
- Austenitic stainless steel: This type of stainless steel contains high levels of chromium and nickel, making it highly corrosion-resistant and durable. It is also non-magnetic and has good formability and weldability. Examples of austenitic stainless steel include AISI 200 and 300 series.
- Ferritic stainless steel: This type of stainless steel contains high levels of chromium, but low levels of nickel. It has good corrosion resistance but is not as durable as austenitic stainless steel. It is also magnetic and has good formability, but limited weldability. Examples of ferritic stainless steel include 430, 444, 409 and 439.
- Martensitic stainless steel: This type of stainless steel contains high levels of carbon and is known for its high strength and hardness. It is also magnetic and has good formability and weldability. However, it is not as corrosion-resistant as austenitic or ferritic stainless steel. Examples of martensitic stainless steel include 410, 420, 431, 440 and 416.
- Duplex stainless steel: This type of stainless steel is a combination of austenitic and ferritic stainless steel, combining the corrosion resistance of austenitic stainless steel with the strength and durability of ferritic stainless steel. It is also non-magnetic and has good formability and weldability. Examples of duplex stainless steel include 2205, 2304 and 2507.
- Precipitation hardening stainless steel: This type of stainless steel is heat treated to increase its strength and hardness. It has good corrosion resistance and is non-magnetic, but has limited formability and weldability. Examples of precipitation-hardening stainless steel include 17-4 PH and 13-8 PH.
Now, let’s have a deeper look inside them.
Austenitic stainless steel castings
Austenitic stainless steel is one of the five classes of stainless steel by crystalline structure (along with the other 4 types we referred to above). Its primary crystalline structure is austenite (face-centered cubic) and it prevents steels from being hardenable by heat treatment and makes them essentially non-magnetic.
Austenitic stainless steel contains at least 10.5 percent and 8 to 12 percent nickel, the structure is achieved by adding enough austenite-stabilizing elements such as nickel, manganese and nitrogen. Chromium is what gives the steel its high corrosion resistance, while nitrogen is a stiffening agent.
There are two subgroups of austenitic stainless steel. 300 series stainless steels achieve their austenitic structure primarily by a nickel addition while 200 series stainless steels substitute manganese and nitrogen for nickel, though there is still a small nickel content.
300 series stainless steels are the larger subgroup. The most common austenitic stainless steel and most common of all stainless steel is Type 304, also known as 18/8 or A2. Type 304 is extensively used in such items as cookware, cutlery, and kitchen equipment. Type 316 is the next most common austenitic stainless steel. Some 300 series, such as Type 316, also contain some molybdenum to promote resistance to acids and increase resistance to localized attacks.
| Euronorm (EN) number | EN designation | AISI grade | C | Cr | Mo | Ni | Others | Melts at | Remark |
| 1.431 | X10CrNi18-8 | 301 | 0.1 | 17.5 | NS | 8 | NS | 1420 | For springs |
| 1.4301 | X5CrNi18-10 | 304 | < 0.07 | 18.5 | NS | 9 | NS | 1450 | A very common austenitic stainless steel grade |
| 1.4307 | X2CrNi18-9 | 304L | < 0.030 | 18.5 | NS | 9 | NS | 1450 | Same as above but not susceptible to intergranular corrosion thanks to a lower C content. |
| 1.4305 | X8CrNiS18-9 e | 303 | < 0.10 | 18 | NS | 9 | 0.3 | 1420 | Sulfur is added to improve machinability. |
| 1.4541 | X6CrNiTi18-10 | 321 | < 0.08 | 18 | NS | 10.5 | Ti: 5×C ≤ 0.70 | 1425 | Same as grade 1.4301 but not susceptible to intergranular corrosion thanks to Ti which “traps” C. |
| 1.4401 | X5CrNiMo17-12-2 | 316 | < 0.07 | 17.5 | 2.2 | 11.5 | NS | 1400 | Second best known austenitic grade. Mo increases the corrosion resistance. |
| 1.4404 | X2CrNiMo17-12-2 | 316L | < 0.030 | 17.5 | 2.25 | 11.5 | NS | 1400 | Same as above but not susceptible to intergranular corrosion thanks to a lower C content. |
| 1.4571 | X6CrNiMoTi17-12-2 | 316Ti | < 0.08 | 17.5 | 2.25 | 12 | Ti: 5×C ≤ 0.70 |
The higher nitrogen addition in 200 series gives them higher mechanical strength than 300 series.
Ferritic stainless steel castings
Ferritic stainless steels are the next most widely used type of stainless steel after austenitic stainless steel. They contain a high proportion of ferrite, which is a solid solution of iron with a body-centered cubic crystal structure. It has a lower melting point and a lower cost compared to austenitic stainless steel. It is also magnetic. By comparison with austenitic types, these are less hardenable by cold working, less weldable, and should not be used at cryogenic temperatures. Some types, like the 430, have excellent corrosion resistance and are very heat tolerant.
They contain 11% to 27% chromium and small amounts of ferrite stabilizers, such as niobium and titanium. Ferritic alloys exhibit ferromagnetic behavior up to a temperature known as the Curie point (650 °C – 750 °C), beyond which materials lose their permanent magnetic properties.
The BCC grain structure of ferritic alloys, which is the same as that of pure iron at room temperature, is the reason for its magnetic nature. Ferritic stainless steels, similar to austenitic stainless steels, cannot be strengthened by heat treatment.
| Euronorm (EN) number | AISI grade | C | Cr | Mn | Ni | Others | Melts at |
| 1.4512 | 409 | 0.06% | 10.5 – 11.7 % | 1% | 0.50% | 1375 – 1450 °C | |
| 1.4016 | 430 | 0.04% | 16 – 18 % | 1% | NS | Si 1% | 1375 – 1450 °C |
| 1.4000 | 405 | 0.08% | 11.5 – 14.5 % | 1% | 0.60% | 1375 – 1450 °C | |
| 1.4113 | 434 | 0.12% | 16 – 18 % | 1% | 1.00% | Mo 0.75 – 1.25 % | 1375 – 1450 °C |
| 1.4521 | 444 | 0.03% | 17.5 – 19.5 % | 1% | 1.00% | Mo 1.75 – 2.5 % | 1375 – 1450 °C |
Ferritic stainless steels are not as resistant to corrosion as austenitic steels, but they still possess very good corrosion and oxidation resistance. They have good resistance to stress-corrosion cracking, and they generally have better engineering properties including ductility and formability than austenitic alloys. Although ferritic alloys are weldable, some problems such as cracking along the heat-affected zones exist.
Martensitic stainless steel castings
Martensitic stainless steel is a type of stainless steel alloy that has a martensite crystal structure. They typically contain between 11.5 to 18% chromium, and up to 1% carbon and other elements, such as nickel, selenium, phosphorus, vanadium, and sulfur are added in different grades to achieve specific properties. These steels have a face-centered cubic (FCC) structure at high temperatures.
They were developed mainly to satisfy the property requirements for hardness, high strength, wear resistance, and corrosion resistance. They are also ferromagnetic, meaning that they can retain their magnetic properties after the magnetic field is withdrawn. Unlike ferritic and austenitic stainless steels, they can be hardened and tempered through aging and heat treatment. However, due to their relatively lower chromium content, martensitic stainless steels are not as corrosion-resistant as ferritic or austenitic stainless steels
Martensitic stainless steels make up the 400 series of stainless steels. The 410 grade is the base grade and also the most commonly used one. It typically contains 11.5 – 13% chromium, 0.15% carbon, and 0.1% manganese and is used in applications such as gas turbine blades and cutlery. 416 is another popular grade. It contains more chromium and manganese with an addition of molybdenum and sulfur/selenium, and it is used to make screws and gears.
| EN designation | Euronorm (EN) number | AISI grade | C | Cr | Mo | Others | Remarks |
| X12Cr13 | 1.4006 | 410 | 0.12 | 12.5 | — | — | Base grade, used as stainless engineering steel |
| X20Cr13 | 1.4021 | 420 | 0.2 | 13 | — | — | Base grade, used as stainless engineering steel |
| X50CrMoV15 | 1.4116 | – | 0.5 | 14.5 | 0.65 | V: 0.15 | Used chiefly for professional knives |
| X14CrMoS17 | 1.4104 | 430F | 0.14 | 16.5 | 0.4 | S: 0.25 | Sulfur improves machinability |
| X39CrMo17-1 | 1.4122 | – | 0.4 | 16.5 | 1.1 | — | Used chiefly for professional knives |
| X105CrMo17 | 1.4125 | 440C | 1.1 | 17 | 0.6 | — | Tool steel grade (440C), high wear resistance |
| X17CrNi16-2 | 1.4057 | 431 | 0.17 | 16 | — | Ni: 2.00 | Ni replaces some C for higher ductility & toughness |
| X4CrNiMo16-5-1 | 1.4418 | – | ≤ 0.06 | 16 | 1.1 | Ni: 2.00 | The highest corrosion resistance of martensitic |
| X5CrNiCuNb16-4 | 1.4542 | 630 (17-4PH) | ≤ 0.07 | 16 | – | Ni: 4.00 Cu: 4.00 Nb: 5xC to 0.45 |
Precipitation hardening grade. High strength. Used in aerospace. |
Duplex stainless steel castings
Duplex stainless steels get their name from their two-phase microstructure, ferrite and austenite in approximately equal measure. While exact ratios vary by grade, most duplex steels have a structure that is roughly 50 percent austenite and 50 percent ferrite. This allows them to benefit from the advantages of both austenitic and ferritic stainless steels, leading to increased strength, improved weldability, higher toughness and resistance to several types of corrosion. Commercially, they are also cheaper than austenitic stainless steels due to their lower nickel content.
At high temperatures, the relatively unstable ferrite phase in duplex steels gets converted into the undesirable α’ (alpha prime) phase, which causes a decrease in their mechanical properties, such as strength and toughness, and also in their corrosion resistance.
The main differences in composition, when compared with austenitic stainless steel are that the duplex steels have a higher chromium content, 20–28%; higher molybdenum, up to 5%; lower nickel, up to 9% and 0.05–0.50% nitrogen. Both the low nickel content and the high strength (enabling thinner sections to be used) give significant cost benefits. They are therefore used extensively in the offshore oil and gas industry for pipework systems, manifolds, risers, etc., and in the petrochemical industry in the form of pipelines and pressure vessels.
In the early stages of the development of duplex stainless steel, there were only a few grades, the most popular among them being duplex stainless steel UNS S31803. Thereafter, the development of new grades started, and they were determined by their end application, which can be categorized into two main types:
- Hyper-duplex and super-duplex stainless steels were created to operate in very corrosive environments but with less focus on their strength
- Lean and standard duplex stainless steels, which had more focus on increased strength and were to be used in mildly corrosive environments such as in structural applications.
These categories are now identified by the pitting resistance equivalence number (PREN) which is calculated by a formula based on the composition of duplex stainless steels.
PREN=%Cr+3.3×%Mo+16×%N
| Steel designation | Number | C, max. | Si | Mn | P, max. | S, max. | N | Cr | Cu | Mo | Ni | Other |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| X2CrNiN22-2 | 1.4062 | 0.03 | ≤1.00 | ≤2.00 | 0.04 | 0.010 | 0.16 to 0.28 | 21.5 to 24.0 | – | ≤0.45 | 1.00 to 2.90 | – |
| X2CrCuNiN23-2-2 | 1.4669 | 0.045 | ≤1.00 | 1.00 to 3.00 | 0.04 | 0.030 | 0.12 to 0.20 | 21.5 to 24.0 | 1.60 to 3.00 | ≤0.50 | 1.00 to 3.00 | – |
| X2CrNiMoSi18-5-3 | 1.4424 | 0.03 | 1.40 to 2.00 | 1.20 to 2.00 | 0.035 | 0.015 | 0.05 to 0.10 | 18.0 to 19.0 | – | 2.5 to 3.0 | 4.5 to 5.2 | – |
| X2CrNiN23-4 | 1.4362 | 0.03 | ≤1.00 | ≤2.00 | 0.035 | 0.015 | 0.05 to 0.20 | 22.0 to 24.5 | 0.10 to 0.60 | 0.10 to 0.60 | 3.5 to 5.5 | – |
| X2CrMnNiN21-5-1 | 1.4162 | 0.04 | ≤1.00 | 4.0 to 6.0 | 0.040 | 0.015 | 0.20 to 0.25 | 21.0 to 22.0 | 0.10 to 0.80 | 0.10 to 0.80 | 1.35 to 1.90 | – |
| X2CrMnNiMoN21-5-3 | 1.4482 | 0.03 | ≤1.00 | 4.0 to 6.0 | 0.035 | 0.030 | 0.05 to 0.20 | 19.5 to 21.5 | ≤1.00 | 0.10 to 0.60 | 1.50 to 3.50 | – |
| X2CrNiMoN22-5-3 | 1.4462 | 0.03 | ≤1.00 | ≤2.00 | 0.035 | 0.015 | 0.10 to 0.22 | 21.0 to 23.0 | – | 2.50 to 3.50 | 4.5 to 6.5 | – |
| X2CrNiMnMoCuN24-4-3-2 | 1.4662 | 0.03 | ≤0.70 | 2.5 to 4.0 | 0.035 | 0.005 | 0.20 to 0.30 | 23.0 to 25.0 | 0.10 to 0.80 | 1.00 to 2.00 | 3.0 to 4.5 | |
| X2CrNiMoCuN25-6-3 | 1.4507 | 0.03 | ≤0.70 | ≤2.00 | 0.035 | 0.015 | 0.20 to 0.30 | 24.0 to 26.0 | 1.00 to 2.50 | 3.0 to 4.0 | 6.0 to 8.0 | – |
| X3CrNiMoN27-5-2 | 1.4460 | 0.05 | ≤1.00 | ≤2.00 | 0.035 | 0.015 | 0.05 to 0.20 | 25.0 to 28.0 | – | 1.30 to 2.00 | 4.5 to 6.5 | – |
Precipitation hardening stainless steel
Precipitation hardening (PH) stainless steel is similar to other stainless steel and nickel-based alloys, with one major exception: It is strengthened by a heat treatment process that involves aging the steel at a high temperature. During this process, small precipitates of hardening elements form within the steel matrix, which increases the strength and hardness of the material.
PH stainless steel is typically made from a combination of austenitic and ferritic stainless steel. The ferritic phase provides corrosion resistance, while the austenitic phase provides formability and ductility. By adding elements such as copper, aluminum, phosphorus, or titanium to the steel and heat treating it, the tensile and yield strength of the material can be significantly increased.
PH stainless steel is known for its high strength and good corrosion resistance, making it suitable for use in various applications, such as aerospace components, defense systems, and automotive parts. It is also resistant to stress corrosion cracking, making it a good choice in high-pressure and high-temperature environments.
The family of precipitation-hardening stainless steels can be divided into three main types – low carbon martensitic, semi-austenitic and austenitic – typical compositions of some of the steels are given in the following table.
| Specification | Common Name | Type | Typical Chemical Analysis % | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| C | Mn | Cr | Ni | Mo | Cu | Al | Ti | Others | |||
| A693 Tp630 | 17/4PH | martensitic | 0.05 | 0.75 | 16.5 | 4.25 | – | 4.25 | – | – | Nb 0.3 |
| FV 520 | austenitic-martensitic | 0.05 | 0.6 | 14.5 | 4.75 | 1.4 | 1.7 | – | – | Nb 0.3 | |
| A693 Tp631 | 17/7PH | austenitic-martensitic | 0.06 | 0.7 | 17.25 | 7.25 | – | – | 1.25 | – | – |
| PH 15/7 Mo | austenitic-martensitic | 0.06 | 0.7 | 15.5 | 7.25 | 2.6 | – | 1.3 | – | – | |
| A 286 | austenitic | 0.04 | 1.45 | 15.25 | 26.0 | 1.25 | – | 0.15 | 2.15 | V 0.25 B 0.007 |
|
| JBK 75 | austenitic | 0.01 | 0.04 | 14.75 | 30.5 | 1.25 | – | 0.30 | 2.15 | V 0.25 B 0.0017 |
|
| 17/10P | austenitic | 0.07 | 0.75 | 17.2 | 10.8 | P 0.28 | |||||
The hardening process generally consists of three main steps. First, the metal must undergo a solution treatment. In this stage, the metal will be heated to a high temperature to dissolve any precipitates and alloying agents into the supersaturated solution. A typical temperature range is 1800° to 1950°F and can be performed in conjunction with a hot rolling process.
Next, the metal will undergo a quenching step to cool it to room temperature. This can be done in air, oil or water at a rate fast enough to cause supersaturation of the solid solution. Slow cooling is more likely to produce a coarser grain size than rapid cooling. In general, the finer the grain size, the better the properties of the finished alloy.
The third step is called precipitation (or age) hardening. The supersaturated solid solution will break down as small clusters of precipitates form, greatly strengthening the metal. For stainless steels, this process involves holding the metal at a constant high temperature for a certain amount of time and then air cooling to room temperature.
Although PH alloys are more complex metallurgically, they are not necessarily more costly than many non-age-hardening alloys. In fact, performance may be substantially higher than that of such alloys without cost loss. Although corrosion resistance decreases (or may increase) during the aging cycle, this is a very small amount.
Resources:
https://en.wikipedia.org/wiki/Austenitic_stainless_steel
https://en.wikipedia.org/wiki/Duplex_stainless_steel
https://www.unifiedalloys.com/blog/what-is-duplex-stainless
https://www.ulbrich.com/blog/precipitation-hardening-stainless-steel-alloys-guide/
https://www.twi-global.com/technical-knowledge/job-knowledge/precipitation-hardening-stainless-steels-102
