Steel is one of the most resilient and versatile materials out there. In fact, its invention was one of the factors accelerating the industrial age.
Steel is an iron alloy – a mixture of iron and other compounds, and stainless steel involves chromium. Adding chromium to iron makes the resulting material resistant to rust and corrosion, giving it a shiny appearance that’s easy to clean.
Stainless steel is available in many grades that differ in the percentage composition of additive materials. You’ll find it in various types, such as austenitic, ferritic, and martensitic ones. In this article, we’ll focus on a type of martensitic stainless steel – grade 431.
This steel features a crystalline martensite structure, which makes the steel very hard and durable.
What Is Stainless Steel 431 Composed of?
This grade of stainless steel contains chromium, nickel, carbon, manganese, silicon, phosphorus, and sulfur mixed with iron.
Stainless steel 431 has more chromium content than other stainless steel grades. While other types have around 11-12% of chromium, stainless steel 431 has around 15-17%. This higher proportion of chromium means that it is more corrosion-resistant and has exceptional toughness and torque strength.
The nickel content in stainless steel 431 is around 1.25-2.5%, considerably lower than austenitic steel, which has around 8-10% of nickel. The alloy also has trace amounts of carbon, manganese, and silicon, which improve its toughness.
An average grade-431 stainless steel comprises less than 0.20% carbon, less than 1% manganese and silicon, around 0.03-0.04% of phosphorus and sulfur, 15-17% of chromium, and 1.25-2.5% nickel mixed with iron.
How Is Stainless Steel 431 Manufactured?
Stainless steel 431 is martensitic stainless steel that you manufacture by rapidly cooling austenite iron in a process called quenching. The cooling should be so rapid that the iron atoms should not diffuse out of the crystal structure.
The process of stainless steel manufacturing uses an electric air furnace (EAF) which uses electricity to melt the metals. It consists of the following steps:
- You start the process by melting all the raw materials like carbon, iron, chromium, and nickel in an electric iron furnace.
- The argon-oxygen decarburization (AOD) process refines the molten metals. You blow argon and oxygen gas into the molten mixture to remove impurities and reduce carbon levels.
- You pour the molten metal into molds to make ingots or billets and leave it to cool.
- You heat and pass the ingots through rollers to shape them into the required form. This process also improves steel strength by refining its grain structure.
- You anneal the steel, which means you heat it, and then cool it slowly. This process improves its machinability.
- You can now cut the steel into the desired shape and size.
Stainless Steel 431 Properties
Stainless steel grade 431 has several valuable properties that make it suitable for various functions. These include:
Resistance to Corrosion
This grade has similar corrosion resistance as grade 304. It is durable in saltwater conditions but less resistant in tropical water conditions.
431 steel is resistant to corrosion in mildly corrosive and marine environments, making it suitable for use in submarines or for turbines in the coastal region. However, if you intend to place your steel object in a highly corrosive or acidic environment, go for austenitic steel instead.
Strength
The carbon and high chromium content gives stainless steel 431 decent durabilities and strength. This property also makes it resistant to wear and tear.
The tensile strength of 431 steel is an impressive 850-1000 MPa, and its hardness is around 20-24 HRC.
Physical and Magnetic Properties
Stainless steel 431 is magnetic, which makes it ideal to be used in electronics and other areas where you require magnetism.
It is less machinable than other steel grades in normal conditions, but you can machine it easily when it is annealed.
You can weld it using standard welding techniques like gas tungsten or shielded metal arc welding, but you must preheat it. You will also require post-weld heat treatment to prevent cracks from forming.
Reactivity
The high chromium content makes this grade relatively non-reactive, as it forms a passive oxide layer on its surface. This layer prevents most corrosion and keeps the material non-reactive.
This steel grade is ideal when you aren’t working with very reactive chemicals, such as in the food industry.
Heat Treatability
Stainless steel 431 is heat treatable, so you can improve its strength and hardness by heating it. This process has two steps, annealing and hardening.
First, you heat your metal slowly. Then, you harden it by heating it at a high temperature and then rapidly cooling it in water. This rapid cooling locks in the martensite structure, which is extra hard and strong.
Uses of Stainless Steel 431
You can use stainless steel grade 431 in many industries. We have listed some common uses here:
Pump Shafts
Pump shafts face mechanical stress, corrosion, and wear and tear regularly. So, you need to use a material with high strength, corrosion resistance, and abrasion resistance properties. As discussed earlier, stainless steel 431 has these properties, making it suitable for the job.
You can also machine this steel easily when you anneal it, so cutting it into a pump shaft’s shape becomes easy.
Turbine Blades
Stainless steel 431 is tough, resists corrosion, and doesn’t wear down easily. These properties are incredibly useful in making turbine blades.
You usually place wind turbines in coastal areas with a saltish marine environment, and stainless steel 431 is particularly resistant to saltwater corrosion. This makes it ideal for the job.
Marine Systems
Stainless steel 431 is particularly resistant to saltwater corrosion, making it suitable for use in marine systems, such as ships’ propeller shafts and marine pump components. These components are constantly exposed to saltwater conditions, requiring high corrosion resistance.
Aerospace Industry
You can use this grade in the aerospace industry, particularly in aircraft fasteners, landing gear, and exhaust components. The aerospace industry requires high strength and wear resistance provided by this steel grade. This grade is also magnetic, making it suitable for electronic systems in the aerospace industry.
Food Appliances
The steel’s chromium component provides a protective oxide layer that protects against weak food acids and provides a non-reactive environment. These properties make it suitable if you are manufacturing food appliances.
Drawbacks of Stainless Steel 431
While stainless steel 431 is beneficial in many situations, it has a few disadvantages:
Pitting Corrosion
While stainless steel 431 is corrosion resistant in mildly corrosive environments, it can corrode in highly corrosive environments, like in the presence of strong acids. They attack the protective oxide layer and create pits or holes.
Low Machinability
While stainless steel 431 is very tough, this results in less machinability: cutting and molding it into desired shapes and sizes is more complicated. However, when annealed, you can machine it comparatively with ease.
High Costs
As steel 431 alloy uses more amounts of expensive materials like chromium, it costs more than most steel grades. However, it is more resistant to damage and can last longer.
Endnote
Stainless steel has many grades, such as stainless steel 431. It is a type of martensitic steel that uses much chromium, giving it its enhanced corrosion resistance properties.
Common areas where this steel grade is used include the marine industry, aerospace industry, turbines, and pump shafts. Its comparatively inert nature makes it suitable for the food industry too. If your industry requires a strong and corrosion-resistant steel grade in a marine environment, consider stainless steel 431.
