There are different types of stainless steel to consider for manufacturing compressors.
While they may all be made of steel, they have some properties that are unique to each grade, and as such, they are most suited for the particular machine and environment they will be used in.
That is why we’ve outlined these stainless steel, their properties, and limitations to give you an insight on which to settle for when it comes to the manufacturing of compressors.
What Is Stainless Steel?
Stainless steel is an alloy of steel which has a maximum of 1.2% carbon by mass and a minimum of 10.5% chromium content by mass.
These alloys are widely known for their high resistivity to corrosion; a resistivity that increases with an increase in their Chromium content and addition of molybdenum.
As a result, stainless steel has become an ideal material in applications where corrosion resistance and high strength are a priority.
How Are the Types of Stainless Steel Classified?
Stainless steels are classified into grades or types based on their chromium and molybdenum contents.
The combination of either of these determines the environment in which the alloy will be used.
That being the case, there are four broad classifications of stainless steel and these are:
- Ferritic stainless steels
- Austenitic stainless steels
- Martensitic stainless steels
- Precipitation-hardening stainless steels
1. Ferritic Stainless Steels:
Ferritic stainless steels can be identified as the T400 series stainless steels (made up of alloys such as T409, T430).
They contain only chromium (sometimes small addition of nickel) and cannot be hardened using heat treatment.
Some properties of Ferritic stainless steels are:
- Magnetic
- Good corrosion resistance
- Low carbon and nickel content
- Good weldability and toughness
2. Austenitic Stainless Steels:
Austenitic stainless steels can be identified as the T300 series (typical alloys T304, T316).
These steels are made up of chromium and nickel and they can be hardened by cold working.
While they are non-magnetic, they can be made magnetic by cold working.
Some properties of Austenitic stainless steels are:
- Non-magnetic
- Good creep resistance
- Good weldability and formability
- Good to excellent corrosion resistance
3. Martensitic Stainless Steels:
Martensitic stainless steels are straight chromium steels that can be hardened using heat treatment.
They are magnetic and corrosion resistant to mild environments and also have good ductility and in some cases, can be heated to tensile strengths up to 200,000 psi.
Some properties of Martensitic stainless steels are:
- Magnetic
- High strength
- High wear resistance
- Limited corrosion resistance
4. Precipitation-hardening Stainless Steels:
Precipitation-hardening stainless steels are identified as T17-4 or T17-7 PH and they contain chromium-nickel.
They gain their name from being hardened by solution treating and aging.
Types of Stainless Steel to Consider for Manufacturing Compressors
The following are the different types of stainless steel which can be used in manufacturing compressors. They are:
- AK Steel Type 316
- AK Steel Type 410
- AK Steel Type 420
- AK Steel 17-7
1. AK Steel Type 316:
This is an austenitic chromium-nickel stainless steel which contains molybdenum, a silvery-white metal which has high resistivity to corrosion. Molybdenum also has the highest melting point of all elements.
That being the case, the characteristics of Type 316 includes its ability to increase the general corrosion resistance against acids such as hydrochloric, acetic, sulfuric, formic and tartaric acids; acid sulfates and alkaline chlorides.
The alloy also provides enhanced strength at elevated temperatures, and it improves the resistance to pitting from chloride ion solutions.
It is also worthy to note that this AK Steel is available in two forms and these are the Types 316 and 316L.
Type 316L as another version is an extra-low carbon version which reduces the harmful effects of carbide precipitation as a result of welding. In such cases, high corrosion resistance is assured.
Type 316L is mostly used in manufacturing:
- Digesters
- Jet engine parts
- Heat exchangers
- Exhaust manifolds
- Valve and pump trim
- Paper and textile processing equipment
- Pharmaceutical and photographic equipment
- Parts exposed to marine atmospheres and tubing
Properties of Types 316 and 316L Stainless Steels
Some of the properties that make Types 316 and 316L stainless steels a great choice in the manufacturing of compressors are:
1. Corrosion Resistance:
In comparison to Type 304 sheets of steel, Types 316 and 316L stainless steels have better corrosion resistance.
For this reason, they are also employed in the textile, paper, and photographic industries thanks to their high resistance to chemicals.
2. Heat Treatments:
These alloys are non-hardenable on heating.
The latter can be attributed to the fewer carbons they contain and as such, they cannot be hardened or tempered on heating.
An advantage to this is that it makes the steel less susceptible to breakage due to its brittle nature (as is the case of hardened steel).
3. Weldability:
Through the use of common fusion and resistance techniques, this type of steel alloy are weldable even though their weldability is poorer than Types 304 and 304L.
2. AK Steel Type 410:
Based on its crystalline structure, this is classified as martensitic stainless steel which is a very hard crystalline structure.
This alloy has good corrosion resistance, high strength, and hardness. As a result, applications that require moderate corrosion resistance but high mechanical use of this alloy.
It is also magnetic in annealed and hardened conditions.
AK Steel Type 410 is mostly used in manufacturing:
- Knives
- Hand tools
- Flat springs
- Kitchen utensils
Properties of AK Steel Type 410
Some properties of the AK Steel Type 410 include:
1. Corrosion Resistance:
This steel is corrosion resistant to water, air, and some chemicals. In the case of the latter, it exhibits appreciable resistance to concentrated sulfuric acid, dilute acetic acid, and naptha.
2. Hardening:
This alloy can be hardened by heating.
Its requirements for annealing (heating to reduce the physical and chemical properties of the material too reduce its hardness and increase ductility), process annealing (intermediate annealing), and hardening are:
- Annealing: Heat slowly to 1500 – 1650°F (816 – 899°C), cool to 1100°F (593°C) in a furnace, air cool.
- Process Annealing: Heat to 1350 -1450°F (732 – 788°C), air cool.
- Hardening: Heat to 1700 – 1850°F (927 – 1010°C), air cool or oil quench.
3. Weldability:
AK Steel Type 410 as martensitic steel means it has limited weldability which can be attributed to its hardenability.
Nonetheless, in the martensitic class of stainless steels, Type 410 has the best weldability.
It, however, has a lower carbon content which takes away the need for a post-weld heat treating.
While its hardenability means the steel will not wear off over time, some consideration has to be given such as preheating to 550°F (260°C) in order to prevent cracking.
3. AK Steel Type 420:
This steel, like the Type 410, falls under the class of martensitic stainless steel.
It also has a corrosion resistance that is comparable to the Type 410 but takes it one step further by providing increased strength and hardness.
While Type 420 is magnetic in annealed and hardened conditions, it has maximum corrosion resistance in fully hardened conditions but not in the annealed condition.
Similarly, this alloy is not used at temperatures exceeding 800°F (427°C) due to
rapid softening.
AK Steel Type 420 is used in the manufacture of applications that require high hardness and good corrosion resistance.
They include:
- Cutlery
- Scissors
- Tapes and straight edges
- Surgical and dental instruments
Properties of AK Steel Type 420
Some properties of this include:
1. Corrosion Resistance:
AK Steel Type 420 comes with a promise in hardened or hardened and stress relieved conditions and that it is the provision of good corrosion resistance in either.
Whether it is corrosion caused by mine water, steam, fresh water, atmosphere, ammonia, mercury, carbonic acid, crude oil, gasoline, and a wide range of others, this alloy is resistant.
2. Heat Treatments:
The conditions required for the heat treatments of this alloy are:
Annealing: For maximum softness, heat uniformly to 1500 – 1650°F (816 – 899°C)
and cool slowly in the furnace.
Process Annealing: Heat to 1350 – 1450°F (732 – 788°C), air cool.
Hardening: Preheat, then heat to 1800 – 1950°F (982 – 1066°C), soak at temperature and air cool or quench in warm oil.
Stress Relieving: Heat at 300 – 800°F (149 – 427°C) for 1 to 3 hours, cool in the air
or quench in oil or water.
3. Weldability:
A property common to the martensitic class of stainless steels is their limited weldability as a result of their hardenability.
On the other hand, this alloy has poorer weldability in comparison to most alloys in the same class.
As a consequence, special consideration has to be given to prevent cold cracking and in this case, by preheating to 550°F (260°C).
In the same vein, considerations should be given to Post-weld heat treatment in order to achieve the required properties.
4. AK Steel 17-7:
AK Steel 17-7 falls under the precipitation hardening class of stainless steels which are notable for their high hardness and strength which comes with a promise of excellent fatigue properties.
In addition, this alloy has good formability, corrosion resistance, and minimum distortion upon being heated.
A combination of these properties has given Steel 17-7 use cases in aerospace applications and in other applications where high strength and good corrosion resistance are required.
For compressors which need excellent properties especially in flat springs at temperatures up to 600°F (316°C), this alloy becomes needful.
Properties of AK Steel 17-7
The key properties of this steel include:
1. Corrosion Resistance:
This alloy is superior to other hardenable chromium types of stainless steels including the Types 410, 420 and 431.
Nonetheless, the chromium-nickel Type 304 is a better competitor to Steel 17-7 even though the latter strives to be just as good in most environments.
2. Weldability:
The precipitation hardening class of stainless steels are known to be weldable using common fusion and resistance techniques.
However, great consideration such as the best heat-treated conditions for welding has to be given in a bid to achieve optimum mechanical properties.
Conclusion
These are the major types of stainless steel to consider for manufacturing compressors and as such, valves, springs, pistons, and other parts can take advantage of each of these.
As stainless steel alloys, they come with high strength and good corrosion resistance to a variety of environments.
However, each is best suited in certain environments and depending on the level of weldability or hardenability required.
