Introduction of stainless steels commonly used in cnc machining

Posted on : February 12, 2022 By  GREFEE

Stainless steel has mirror-like brightness, as well as hard and cold sensation. It is an avant-garde decorative material with diverse using advantages, like orrosion resistance, formability, compatibility and toughness.

Stainless steel is used in many industries such as heavy industry, light industry, household goods and architectural decoration.

StaStainless acid-resistant steel is what we called stainless steel. It contains stainless steel and acid-resistant steel whose functions are atmospheric corrosion or chemical corrosion resist respectively.

Carbon has a small proportion in the content of most stainless steels as when the carbon content is higher, the corrosion resistance of stainless steel will be weaken. The carbon content (Max) -1.2% and this figure could be less than 0.03% in some steels (e.g. 00Cr12). Cr(Chromium) has the largest proportion in stainless steel. Its corrosion resistance comes into effect only with a proper level of Cr content.

Therefore, Cr(chromium) content of stainless steel is generally at least 10.5%. Stainless steel also contains Ni, Ti, Mn, N, Nb, Mo, Si, Cu and other elements. Generally speaking, steels will have features of stainless steel when the chromium content(Cr) is higher than 12%. According to the microstructure after heat treatment, stainless steel can be divided into five categories: ferritic stainless steel, martensitic stainless steel, austenitic stainless steel, austenitic ferrite duplex stainless steel and precipitate hardened stainless steel.            

Property: rust resistance, acid resistance

Category: austenitic stainless steel, ferritic stainless steel, martensitic stainless steel ….

Basic information:

Stainless steel has excellent properties of corrosion resistance, formability, compatibility and toughness in a wide temperature range, so it is widely used in industries of heavy industry, light industry, household goods and building decoration.

Classification:

Slassify according to chemical composition, metallographic structure, corrosion resistance, functional features, and basic structure

Among multiple stainless steels classification ways, the metallographic organization is being used mostly. Other ways include:

Chemical composition: basically can be divided into two major systems. That’s to say, chromium stainless steel (e.g. ferrite series, martensite series) and chromium nickel stainless steel (e.g. austenite series, abnormal series, precipitation hardening series).

Metallographic organization: it can be divided into austenitic stainless steel, ferritic stainless steel, martensitic stainless steel, duplex stainless steel, and precipitation hardening stainless steel.

Classify by corrosion resistance types:

It can be divided into stress corrosion resistant stainless steel, pitting corrosion resistant stainless steel, and intergranular corrosion resistant stainless steel, etc.

Classify by functional characteristics:

It can be divided into easy-cutting stainless steel, non magnetic stainless steel, low temperature stainless steel, and high strength stainless steel.

It is estimated that with about 100 types of stainless steels existing in earth. Due to our progress in the area of science, technology, all walks of life are benefited, as well as stainless steels. More and more brands of stainless steel appear. These valuable resource enable us to obtain proportion of the chromium equivalent [Cr] and nickel equivalent [Ni] with different chemical contents information. It provides an access for us to find out more information about their estimated structures and properties with the structure diagram in Schaeffler-Delong.

Basic composition:

Ferritic stainless steel

Chromium content is 12%～30%. When the chrome content increases, its corrosion resistance, toughness and weldability will also increase.

 Chloride stress corrosion resistance is better than that of other stainless steels. Ferritic stainless steels include Crl7, Cr17Mo2Ti, Cr25, Cr25Mo3Ti, Cr28….

Ferritic stainless steels have high chrome content, so they have good properties of corrosion resistance and oxidation resistance, but have relatively poor mechanical property and processing property. Ferritic stainless steels are mostly used in acid-resistant structure without large stress and used as for oxidation resistance steel.

This kind of steel can resist the corrosion of atmosphere, nitric acid and saline solution, and has the characteristics of high temperature oxidation resistance, low thermal expansion coefficient, so they are used in nitric acid and equipments in food factory, and also used to make parts working at high temperature, like gas turbine parts.

Austenitic stainless steel

It contains more than 18% chromium, and also around 8% nickel and a small amount of molybdenum, titanium, nitrogen and other elements. Good comprehensive performance, and can resist the corrosion of a variety of media.

Commonly used brands of austenitic stainless steels are 1Cr18Ni9, 0Cr19Ni9, etc. 

The Wc of 0Cr19Ni9 steel is <0.08%, and the steel number is marked as “0”. This kind of steel contains large amounts of Ni and Cr, making the steel austenitic at room temperature. Austenitic stainless steels have good properties of plasticity, toughness, weldability, corrosion resistance and non-magnetic or weak magnetic. 

They have good corrosion resistance in oxidizing and reducing medium, so they can be used to make acid-resistant equipments, such as corrosion resistant containers, equipment lining, pipelines, nitric acid resistant equipment components, etc., besides, they also can be used to make the main body of stainless steel watches and clocks.

Austenitic stainless steels generally adopt solution treatment, that’s to say, heat the steel to 1050~1150℃, and then water cooling or air cooling it to get the single-phase austenite structure.  

Austenite-ferrite duplex stainless steel

It has the advantages of austenitic and ferritic stainless steels, and has superplasticity. Austenite and ferrite are around fifty-fifty in stainless steel.

Under the condition of low carbon content, the content of Chromium (Cr) is 18%-28%, and the content of nickel (Ni) is 3%-10%. Some steels also contain Mo, Cu, Si, Nb, Ti, N and other alloying elements, which have the characteristics of austenitic and ferritic stainless steels.

Compared with ferrite, this kind of steel has higher plasticity and toughness, no brittleness at room temperature, and has significantly better properties of intergranular corrosion resistance and welding, also has the 475℃ brittleness of ferritic stainless steel and high thermal conductivity, and superplastic property. 

Compared with austenitic stainless steel, it has higher intensity and better property of resistance to intergranular corrosion and chloride stress corrosion. Duplex stainless steel has excellent corrosion resistance, and it’s also a nickel-saving stainless steel.

Martensitic stainless steel

High intensity, and relatively poor plasticity and weldability. Common brands of martensitic stainless steels are 1Cr13, 3Cr13, etc. Since its high content of carbon, it has relatively high intensity, hardness and wear resistance, but has slightly poor corrosion resistance.

This kind of steel is used to make components require high mechanical performance but ordinary corrosion resistance, such as springs, steam turbine blades, hydraulic valves, etc… This kind of steel works after quenching and tempering, and requires annealing after forging and stamping.

Precipitate hardened stainless steel

Its matrix is austenite or martensite. Commonly used brand of precipitate hardened stainless steel is 04Cr13Ni8Mo2Al. It can be hardened by precipitation hardening (also called aging hardening) treatment to achieve hardening (strengthening).

Category

Austenitic stainless steel: stainless steel with austenitic structure at room temperature. This steel has stable austenite structure when it contain s around 18% of Cr, 8%~10% of Ni and around 0.1% of C. 

The austenitic Cr-Ni stainless steels include the famous 18Cr-8Ni steel and the high Cr-Ni steel series developed by increasing content of Cr and Ni and adding Mo, Cu, Si, Nb, Ti, etc.

Austenitic stainless steel is non-magnetic and has high toughness and plasticity, but has relatively low intensity. It can not be strengthened by phase transformation, only can be strengthened by cold processing.

If add S，Ca，Se，Te, austenitic stainless steel will have good property of easy-cutting. If this kind of steel contains Mo and Cu, it will also have properties of corrosion resistance to sulfuric acid, phosphoric acid and formic acid, acetic acid, and urea besides oxidation acid corrosion.

If it contains carbon below 0.03% or contains Ti or Ni, its property of intergranular corrosion resistance can be significantly improved. High silicon austenitic stainless steel has good corrosion resistance to nitric acid.

Austenitic stainless steel has been widely used in all walks of life because its comprehensive and good performance.

Ferritic stainless steel: it’s a stainless steel that ferrite is the main structure in working condition. The content of Cr is around 11%-30%. It has body-centered cubic crystal structure. 

This kind of steel generally has no content of Nickel, with a small amount of Mo, Ti, or Nb sometimes. Features are large thermal conductivity, small expansion coefficient, good oxidation resistance, and excellent stress corrosion resistance.

It’s usually used to make components requiring resistance performance to atmospheric, steam, water and oxidizing acid corrosion.

However, this kind of steel still have weakness, such as weak plasticity, plasticity and corrosion resistance significantly decreased after welding, which limits the application of this steel, but thanks to the application of AOD or VOD greatly decreasing carbon and nitrogen, ferritic stainless steels are widely used now.

Austenite-ferrite duplex stainless steel: a stainless steel that austenite and ferrite are around fifty-fifty. In the condition of low carbon content, the content of Cr is 18%-28%, Ni 3%-10%, some also have Mo, Cu, Nb, Ti, or N.

This kind of steel has features of both austenitic and ferritic stainless steel. Compared with ferritic stainless steel, it has higher plasticity and toughness, no brittleness at room temperature, and has significantly better properties of intergranular corrosion resistance and welding, also has the 475℃ brittleness of ferritic stainless steel and high thermal conductivity, and superplastic property.

Compared with austenitic stainless steel, it has higher intensity and better property of resistance to intergranular corrosion and chloride stress corrosion. Duplex stainless steel has excellent corrosion resistance, and it is also a nickel-saving stainless steel.

Martensitic stainless steel: a stainless steel that its mechanical performance can be adjusted by heat treatment, generally speaking, it’s a stainless steel can be hardened. Typical brands of martensitic stainless steels are Cr13, such as 20Cr13,30Cr13,and 40Cr13.

It has relatively high hardness after quenching, and can achieve different toughness by different tempering temperatures   .

This steel is mainly used to make steam turbine blades, tableware, or surgical devices. Martensitic stainless steels can be divided into martensitic chromium steel and martensitic cr-Ni steel according to the difference of their chemical composition.

And according to structure and strengthening mechanism, martensitic stainless steels can also be divided into  martensitic stainless steel, martensitic and semi-austenitic (or semi-martensitic) precipitation hardening stainless steel and martensitic aging stainless steel.

Representative steel number, chemical composition, and mechanical properties at room temperature: the representative steel numbers of martensitic stainless steel are 12Cr13, 20Cr13, 30Cr13 etc…, ferritic stainless steel are 10Cr17, 10Cr17Mo etc…, austenitic stainless steel are 06Cr19Ni10(304)、022Cr19Ni10(304L), 06Cr17Ni12Mo2(316), 022Cr17Ni12Mo2(316L), 06Cr18Ni11Ti(321), 06Cr18Ni11Nb(347) etc…, and duplex stainless steel is 00Cr26Ni7Mo2Ti.

Comparison and density table for standard stainless steels

Main properties of stainless steel

Weldability: products used in different areas requiring different weldability. Generally there’s no welding requirements on tableware(A class), even including some pot industry.

Besides these products, most ones require raw materials with good performance of welding, such as tableware(B class), thermos cup, steel pipe, water heater, and water dispenser, etc.

Corrosion resistance: most products made in stainless steel requiring good corrosion resistance, such as tableware(A and B class), kitchenware, water heaters, and water dispensers, etc. 

Some foreign customers also test the product performance of corrosion resistance: heat sodium chloride solution to boiling, pour out the solution, after spell, clean and dry up, and then calculate the weight loss to confirm the degree of corrosion (note: in polishing, the product surface may have rust pots in testing since the sand cloth or sandpaper contains the composition of Fe). 

When the content of chromium atom in steel is not less than 12.5%, the electrode potential of steel will change from negative potential to positive electrode potential to avoid electrochemical corrosion.

Polishing performance: nowadays, products made in stainless steel are generally polished in production, only a few products such as water heater, water dispener liner not requiring polishing, which requires the raw materials with good polishing performance.

Main factors affecting the polishing performance are as follows:

• Raw materials have surface defects. Such as scratch, pit, and pickling, etc
• Raw material problems. Too low hardness makes polishing not so bright(not good BQ property), and causes orange grains on product surface in stretching so as to influent BQ property. If raw materials have high hardness, their BQ property would be better.
• After deep stretching, product surface with large distortion may have tiny black spots and ridging, so as to influence the BQ property.

Heat resistance:

heat resistance refers to the stainless steel can still keep its excellent physical and mechanical properties under high temperature.

Influence of carbon: carbon is an element that strongly forms, stabilizes and enlarges the austenite in austenitic stainless steel. The ability of carbon forming austenite is about 30 times that of nickel.

Carbon is a interstitial element, and can significantly improve the intensity of austenitic stainless steel by solution strengthening. Carbon can also improve the austenitic stainless steel’s property of stress and corrosion resistance in high concentration chloride (e.g. 42%MgCl2 boiling solution).

However, carbon is usually regarded as a harmful element in austenitic stainless steel, which mainly because that carbon forms high chromium carbon compounds(Cr23C6) with chromium in steel, which causes chromium depletion in some areas and makes the corrosion resistance of steel decrease(especially intergranular corrosion resistance) in some conditions of making use of its corrosion resistance(such as welding or heated by 450-850℃).

Therefore, since 1960s, most newly developed chrome-nickel austenitic stainless steels contain carbon lower than 0.03% or even 0.02%(ultra-low carbon). We can know that the sensitivity of intergranular corrosion of steel decreases with the decreasing of carbon content, and present the most obvious effect when the carbon content is lower than 0.02%.

Some experiments also point out that carbon also increases the pitting tendency of austenitic stainless steels. As carbon is harmful, we need not only strictly control the carbon content as low as possible in the process of smelting austenitic stainless steel, and also need to prevent adding carbon on the surface of stainless steel and avoid chromium carbide precipitation in the process of cold machining, hot machining and heat treatment.

Main stainless steel products: precipitation hardening stainless steel

Has good formability and weldability, can be used as ultra-high intensity materials and applied in industries of nuclear, aviation and aerospace.

According to its composition, it can be divided into Cr series (400 series), CR-Ni series (300 series), CR-Mn-Ni series (200 series), heat-resistant chromium alloy steel (500 series) and precipitation hardening system (600 series).

200 series: CR-Mn-Ni

201, 202: manganese replaces nickel, has relatively weak corrosion resistance. They are widely used as cheap substitute for 300 series in China.

300 series: CR-Ni austenitic stainless steel

301: good ductility, used for molding products, can be quickly hardened by mechanical processing, good weldability, wear resistance and fatigue strength are better than that of SS304.

302: similar corrosion resistance to SS304, better intensity than SS304 as it contains higher carbon.

303: adding a small amount of sulfur and phosphorus makes SS303 easier to be machined than SS304.

304: general type, that is 18/8 stainless steel. Products made in SS304 are such as: corrosion-resistant containers, tableware, furniture, railings, medical devices. The standard ingredient of SS304 is 18 % chromium and 8 % nickel. It’s a non magnetic stainless steel that metallographic structure can not be changed by heat treatment. GB brand is 06Cr19Ni10.

304 L: has similar property to SS304, has lower content of carbon, so SS304L has better performance of corrosion resistance and easier to be heat treated, but has relatively poor mechanical property, suits for product requiring welding and heat treatment.

304 N: has similar property to SS304, it’s a stainless steel containing nitrogen, and adding nitrogen is to improve the intensity of steel.

309: compared to SS304, SS309 has better temperature resistance, up to 980℃.

309S : high content of chromium and nickel, so has better heat resistance and oxidation resistance, products are such as: heat exchanger, boiler components, jet engines.Has excellent temperature and oxidation resistance, can work under the temperature of 1200℃(highest).

316: besides SS304, SS316 is the second most widely used steel, and mainly used in food industry, clock accessories, pharmaceutical industry and surgical devices. Adding molybdenum makes SS316 get a special structure can resist to corrosion. Compared with SS304, SS316 has better resistance to chloride corrosion, so it’s also used as “Marine steel”. SS316 is usually used in nuclear fuel recovery package. And 18/10 grade stainless steel is also qualified for this application level.

316 L: has lower content of carbon, so SS316L has better performance of corrosion resistance and easier to be heat treated. Products are such as: chemical machining devices, nuclear power generator, refrigerant storage.

321: has similar property to SS304, except that adding titanium decreases the risk of welding corrosion of the material.

347: add the stabilizing element niobium, suits for welding, aerospace parts and chemical equipments.

400 series: ferritic and martensitic stainless steel, manganese-free, can replace SS304 in some degree.

408: good heat resistance, weak corrosion resistance, contains 11% Cr and 8% Ni.

409: the cheapest stainless steel type(UK&US), usually used to make automobile exhaust pipes. It’s a ferritic stainless steel (chrome steel)

410: martensite (high intensity chromium steel), good wear resistance, but poor corrosion resistance.

416: adding sulfur improves the machining property of the material.

420: “Edge grade” martensitic steel, similar to brinell high chromium steel(the earliest stainless steel). It’s also used to make surgical knives, with very bright appearance.

430: ferritic stainless steel, can be used for decorative purposes, such as automobile accessories. Has good formability, but poor temperature and corrosion resistance.

440: high intensity cutting steel, contains relatively high carbon, and can achieve high yield strength after appropriate heat treatment, and its hardness can meet 58HRC, which makes SS440 become the one of the most hardest stainless steel. The most common example is the “razor blade”. Three common types are :440A, 440B, 440C, and 440F(easy-machining).

500 series: heat resistant chromium alloy steel

600 series: martensite precipitates hardened stainless steel

Finish grades of stainless steel

NO.1: the surface is heat treated and pickled after hot rolling. It’s generally used for cold rolled materials, industrial tanks, and chemical industrial equipments, with thickness 2.0mm-8.0mm.

NO.2D: the surface is heat treated and pickled after cold rolling. Material is soft, and surface is silver gloss, usually is used for deep stamping, such as automobile components, and water pipes.

NO.2B: the surface is heat treated and pickled after cold rolling, and then precision rolling to achieve moderate brightness. Smooth surface makes it easy to grind and achieve bright appearance. It’s widely used, such as tableware and building materials. It almost suits for all uses after adopt the surface treatment to improve mechanical properties.

NO.3: products are grinded by #100-#120 sand. It has good gloss and discontinuous coarse grains, and is used for building interior and exterior decoration materials, electrical products and kitchen equipments.

NO.4: products are grinded by #150-#180 grinding belt. It has good gloss and discontinuous coarse grains(thinner than No.3), and is used for bath, building interior and exterior decoration materials, electrical products, kitchen and food equipments.

#320: products are grinded by #320 grinding belt. It has good gloss and discontinuous coarse grains(thinner than No.4), and is used for bath, building interior and exterior decoration materials, electrical products, kitchen and food equipments.

HL: products get grinding riffles after continuous grinding of a strip of reasonable sized polishing sand(subdivision #150-#320). Mainly used for building decoration, elevator, doors, and panels.

HL: products get grinding riffles after continuous grinding of a strip of reasonable sized polishing sand(subdivision #150-#320). Mainly used for building decoration, elevator, doors, and panels.

Conclusion:

compared with aluminum alloy and plastics, stainless steel is more harder to machine, so it has high requirements on machining equipments.

GREFEE has outstanding advantages on manufacturing of stainless steel products as we have many imported advanced CNC machining devices for stainless steel, and products made in GREFEE not only have excellent smooth surface, but also can meet high precision requirements on dimensions.

To achieve high quality, we constantly optimize the processing technology and update the equipments to meet the market needs. We are confident to make your products and challenge more difficulties.

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