Where you need strength and corrosion-resistant properties to co-exist, several conventional stainless steels fall short. Luckily, this gap is where the 555 stainless steel stands out.
But what exactly is this stainless steel, and how can you use it? Read on and learn more.
What Is 555 Stainless Steel?
There are many things you should know about the 555 stainless steel. First, you should remember that it is not a standard international stainless steel grade like 304. It is, instead, a manufacturer-designated stainless steel. Because it is not a standard, its composition varies.
The material is also called “custom 555® stainless steel.”
The 555 stainless steel is a nickel-rich, precipitation-hardening martensitic stainless steel. It is one of the most common stainless steel alloy materials available (think of several of your kitchen equipment).
Most manufacturers of the material will supply them to you in a solution-treated condition too. Then, you can age it via heat treatment to achieve your desired mechanical properties.
To crown it, the material combines and balances strength, toughness, and corrosion resistance in one! The material has an exceptional load-bearing ability. It does not sacrifice strength for durability or environmental resistance.
How Did 555 Stainless Steel Come About?
Engineers developed the first set of PH alloys to ease the treatment of stainless steel through cold working. They also hoped to improve many other qualities. However, there was still a problem.
These PH stainless steel alloys, although excellent to work with, were also prone to cracks in stressful environments. They had a narrow tolerance to damage too. These made them not 100% ideal for fracture-stable aerospace and energy components.
To curb these problems, you need a kind of stainless steel grade that is as tough as it is strong. The same material should be able to withstand stresses that can cause cracks and must be more reliable when you use it to make thick machine components. From these, the stainless steel alloy material, 555 SS, came into being.
Unlike common PH grades like the 17-4 PH, 555 stainless steels are made for high-stress environments. They work for safety-critical services, hence, they are often preferred for specialized services. These specialized services include landing gear systems, structural aircraft components, and high-pressure equipment.
Properties of 555 Stainless Steel
All properties and composition of the 555 stainless steel are:
Physical Properties
- Easy to fabricate
- Good ductility and hardness
- Excellent strength-to-impact ratio
- Better pitting and intergranular corrosion resistance than the average austenitic steels
- Non-sensitive to stress and corrosion cracking.
- High thermal conductivity
Chemical Composition
The chemical elements that are carefully combined in precise amounts to make up the 555 stainless steel are:
| Chemical Element | Percentage Composition (%) | Why? |
| Chromium | Around 11.99 – 20 | Provides the corrosion-resistant property of the alloy |
| Nickel | 6.99–8.0 | Impact strength, toughness, and stability |
| Copper | 0.49–1.5 | Strength through precipitation hardening |
| Molybdenum | 0.99–2 | Increases the microstructural strength during heat treatment, and prevents pitting |
| Manganese | 0.9–1.1 | |
| Silicon | 1.1 | Deoxidant |
| Niobium | ~0.4 | |
| Phosphorus | ~0.035 | Impurity limit |
| Titanium | ≥0.12 | For precipitation control |
| Carbon | ≤0.07 | For toughness and corrosion resistance |
| Iron | Balance |
Mechanical Properties
After aging by heat treatment, the mechanical properties of the 555 stainless steel are highlighted below:
- Exceptional tensile strength that can reach ~1,900 MPa (≈ 275 ksi)
- High yield strength up to ~1,700 MPa (≈ 245 ksi) or greater
- Elongation ratio ~7–10%
- High fracture toughness
- Hardness: varies by the condition you age them (around 44–50 HRC)
Unlike many ultra-high-strength alloys, 555 stainless steel maintains its resistance to cracks even in heat. This is why the alloy is the most suitable for your fracture-critical designs.
Heat Treatment and Aging Conditions of 555 Stainless Steel
Before supplying to you and others who may need the metal, the 555 stainless steel is solution-treated. This solution treatment is done at over ~1000°C high temperature. This degree of heat, as it melts the alloy, dissolves all the alloying elements into a homogeneous austenitic alloy mix.
After heat treatment to age the alloy, the next step is to cool the homogeneous mix rapidly. Rapid cooling changes the material into a martensitic matrix. An alloy with a martensitic structure is better for precipitation hardening.
The first aging you do to the material is important for many reasons. Primarily, it helps you achieve uniform mechanical properties. You also prevent precipitate formation and get better results whenever you age the alloy in the future.
Like other PH stainless steel alloys, you can age 555 stainless steel at different temperatures too. This helps to improve its properties for your use.
Aging the alloy at a lower aging temperature improves its tensile and yield strength. However, the toughness and ductility are lower. On the other hand, when you age at a higher aging strength, you improve the fracture toughness and its ductility. However, your alloy product has a smaller tensile and yield strength.
The aging flexibility of 555 allows you to choose a condition that aligns with your load profile and safety needs. And because the 555 stainless steel is used in life-critical systems, accurate heat-treatment is almost non-negotiable. A slight change in heat and temperature, if not traced prior, reduces toughness. It also compromises the fatigue life of the alloy.
Corrosion Resistance
One reason 555 stainless steel stands out is that it offers good corrosion resistance in different environments. For your use in atmospheric, marine, and mildly aggressive chemical environments, the alloy is ideal. The chromium content protects it against passive film formation, while molybdenum prevents pitting.
Although the alloy does not match the corrosion resistance of fully austenitic stainless steels, it outperforms carbon and low-alloy steels by far.
Applications of 555 Stainless Steel
Common applications of the 555 stainless steel alloy are:
- It is used to make various kitchen utensils and high-grade cookware, pots, sauce pans, kettles, etc.
- Used to make the gear components of airplanes like the landing gear cylinders, structural pins, and shafts. The material is also used to make other load-bearing support elements.
- Its strength makes it an ideal choice to make critical fasteners, drive shafts, and load-transfer components for everyday machines.
- In energy applications, 555 stainless steel is selected to make high-pressure valves, connectors, and downhole components.
Advantages and Limitations of 555 Stainless Steel
555 stainless steel has the following advantages
- It can achieve ultra-high strength levels without excessive brittleness.
- Superior safety margins
- Better resistance to crack initiation and propagation
- Reliable in harsh operational environments
- Reduced failure risk when used in heavy machining
- Maintain its properties within tight tolerances
- Easy to clean, and has a scratch-resistant surface
- Extended service life
The limitations of 555 stainless steel are:
- It is more expensive to process than other traditional PH stainless steels
- It requires a more complex heat treatment during processing for the best results
Machining and Processing 555 Stainless Steel
To get the best results, it is best that you machine the alloy before you age it. This will reduce the rate of tool wear and avoid work hardening.
You may also weld the material yourself. But like other PH materials, you must control the heat you use and choose proper fillers. Although it rarely cracks, you still need to use skilled procedures to avoid cracking.
After welding, giving the material a post-weld heat treatment is essential. This treatment will ensure the integrity of the alloy’s structure and relieve its residual stresses. Further treatment restores its mechanical properties.
Conclusion
555 stainless steel has a better balance of qualities than most precipitation-hardening stainless steels. So where you don’t want a material failure, 555 stainless steel has a commendable performance. Its reliability is excellent, so its long-term value for your safety applications is recommended.
FAQs
Is 555 stainless steel magnetic?
Its magnetic ability depends more on its composition. But, as a martensitic precipitation-hardening stainless steel, 555 stainless steel is magnetic.
Is 555 stainless steel stronger than 17-4 PH?
Yes, you can say so. 555 stainless steel offers higher strength and superior fracture toughness compared to 17-4 PH.
Can 555 stainless steel be welded?
Yes, you can weld the alloy yourself. However, you need to get your welding skills in place as welding needs controlled procedures and mandatory post-weld heat treatment. That way, you don’t lose its properties.
Is ACX 555 the same as 555 stainless steel?
No, both the ACX 555 and 555 stainless steel grades are different alloys. The ACX 555 is a defined stainless steel grade produced by some organizations. Its properties correspond to stainless steels with EN steel number 1.4521 or AISI 444.
555 stainless steel is not a standard metallurgical stainless steel alloy grade. It is used to make cookware, etc.
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