Is 304 stainless steel medical grade?

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Compared with industrial stainless steel, medical stainless steel has more strict requirements on chemical composition because of its main properties of reducing metal ion dissolution and avoiding local corrosion such as intergranular corrosion and stress corrosion. The content of alloy elements such as Ni and Cr is higher than that of ordinary stainless steel (usually the upper limit of ordinary stainless steel), while the content of impurity elements such as S and P is lower than that of ordinary stainless steel. For years, medical stainless steel has been the preferred material for surgical applications, especially in critical care and surgery situations. The element Ni and Cr feature higher corrosion-resisting, which allows it to be used for purposes where orthopedic implants, oral cavity, medical devices are required. Stainless steel,  a type of Ni-Cr alloys, offering a variety of benefits when compared with general grade stainless steel. The type of alloy used in medical stainless steel used in surgical instruments is crucial to the instrument’s ability to resist corrosion and remain free of internal errors and gaps.

Many stainless steels can be used for medical purposes, the most common of which is Austenitic 316 (AISI 316L), known as “surgical steel”. AISI 301 is the most commonly used metal for the manufacture of medical springs. Other commonly used stainless steels for medical use include 420, 440, and 17-4PH. These Martensitic stainless steels are not as resistant to corrosion as Austenitic stainless steels 316, but they have higher hardness. Therefore, Martensitic stainless steel plants are used for cutting tools or other non-implant devices. The gains elasticity in cold work but loses corrosion resistance. Medical stainless steel has achieved widespread popularity due to its unmatched durability, heat treatment resistance, surgical functionality, and corrosion resistance. It is used in a variety of applications including hospital seating frames, cradles, end plates, surgical gloves, IV poles, and staples. Due to its extreme resilience and the need for its use in specialty applications, it is imperative that manufacturers using this grade of stainless steel pay close attention to quality control and manufacturing specification. The most popular medical stainless steel used in the manufacture of surgical instruments are 304 and 316. However, the best alloys feature a lower carbon content and Mo added like 316L and 317L steel.

304 stainless steel, namely 18-8 stainless steel, 304 series stainless steel also includes lower carbon 304L, 304H for heat-resistant purposes, there is a question, 304 stainless steel can be used for medical purposes? There is a fact that in 1926,18% CR-8% Ni stainless steel (AISI 304) was first used as an orthopedic implant material and later in stomatology. It was not until 1952 that AISI 316 stainless steel containing 2%Mo was used in the clinic and gradually replaced 304 stainless steel. In order to solve the intergranular corrosion problem of stainless steel, in the 1960s, the ultra-low carbon stainless steel AISI 316L and AISI 317L with good biocompatibility, mechanical properties and better corrosion resistance began to be used in the medical field. However, Ni is a potential sensitization factor to the human body. In recent years, many countries have limited the content of Ni in daily necessities and medical metal materials, and the maximum allowable Ni content is becoming lower and lower. The Standard 94/27/EC of the European Parliament promulgated in 1994 requires that the Ni content in the materials implanted in the human body (implant materials, orthodontic dentures, etc.) should not exceed 0.105%; For metal materials (jewelry, watches, rings, bracelets, etc.) that are exposed to human skin for a long time, the maximum amount of Ni should not exceed 015Lg/cm2 per week. Today 304 is still used in the manufacture of common medical instruments such as syringes, medical scissors, tweezers, and scalpel series.

 

Difference between 2B and 2D stainless steel sheet

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Stainless steel has become a widely used metal material for its excellent corrosion resistance, good mechanical properties and machining properties. Different processing methods and cold rolling after processing, stainless steel surface can have different levels of surface finish, grain and color. The surface processing of cold-rolled stainless steel plate has 2D, 2B, No.3, No.4, 240, 320, No.7, No.8, HL, BA, TR hard state, embossed surface grade. It can be further applied to electroplating, electropolishing, undirected hairline, etching, shot peening, coloring, coating and other deep-processing surfaces based on the cold-rolled stainless steel. Stainless steel cold rolled sheet is widely used in construction, decoration, home appliances, rail transportation, automobile, elevator, container, solar energy, precision electronics and other fields, including construction, decoration, elevator, container and other products directly use 2D, 2B, BA, grinding and another surface after cold rolling processing, and household appliances, rail transportation, automobiles, solar energy, precision electronics and other industries often use direct processing of cold-rolled stainless steel plate or shallow grinding and polishing stainless steel plate.

 

No.2D stainless steel sheet

No.2D is a kind of cold-rolled dull surface without an oxide scale. After cold rolling, it only goes through heat treatment and pickling. The brightness of its surface is determined by the degree of deformation of cold rolling and the finish of the work roll surface of the finished product pass, and it is also related to the pickling method of removing oxidation. No.2D surface also includes a rough surface roller for light leveling on the above basis. The rough surface roll is a special process to coat the surface of the roll, that is, a number of phase change hard particles are formed on the surface of the roll, and the uneven surface structure is realized on the surface of the steel plate during the leveling process. This kind of surface is suitable for the deep drawing forming process, can improve the friction and contact condition between steel plate and die, is conducive to material flow, improve the forming quality of the workpiece. No.2D surface stainless steel is widely used in building curtain walls, especially those parts of the building that do not require reflection. The roughness Ra of the surface measured by the instrument is about 0.4 ~ 1.0μm.

 

No.2B stainless steel sheet

The most difference between No. 2B and the 2D surface is that No 2B have a smooth process of leveling roll, it looks more light compared with the 2D surface, the instrument measuring the surface roughness of the Ra value is 0.1 ~ 0.5 mu m, is the most common process and have the most extensive application, suitable for the chemical industry, papermaking, oil, medical and other general purposes, also used for building wall.

Appearance

 

 Features Colour Process Applications
NO.2D The surface is even and matte Shiny silver white

 

 Hot rolling + annealing shot peening pickling + cold rolling + annealing pickling 2D is suitable for non-strict surface requirements, general purposes, deep stamping processing, such as automotive components, water pipes, etc
NO.2B More gloss than NO.2D Silvery white with better gloss and finish than 2D surfaces Hot rolling + annealing peening pickling + cold rolling + annealing pickling + quenching and tempering rolling.NO.2D treatment is followed by a final mild cold rolling with a polishing roller, which is the most commonly used surface finish General applications such as tableware, building materials, etc.

 

 

 

What’s the 8K mirror stainless steel sheet?

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Because of its unique corrosion resistance, good processing performance and exquisite surface appearance, stainless steel has been widely used in many fields such as aerospace, energy, military, construction, petrochemical and so on. Polishing is an important part of the stainless steel plate in the decoration industry, its purpose is to get the final mirror (8K) stainless steel. 8K surface (No.8) is the mirror-polished surface, high reflectivity, clear reflection image, usually with resolution and surface defect rate to measure the quality of mirror stainless steel, general visual assessment: level 1 is the surface bright as a mirror, can clearly see the human features and eyebrows; Level 2 is the surface is bright, can see the human features and eyebrows, but the eyebrow part is not clear; Level 3 is good surface brightness, can see the person’s facial features and outline, eyebrow part blurred; Level 4 is the surface gloss, but can not see the person’s facial features; Grade 5 is gray and dull surface.

 

The stainless steel mirror plate is through the mirror polishing of the initial surface of the stainless steel plate BA, 2B or No.1 polishing to become similar to the mirror surface (scientific name 8K mirror or No.8). The mirror steel plate is the substrate for processing subsequent color or etched plates. mainly used in all kinds of decoration or metal optical products. The corrosion resistance of stainless steel depends on its alloy composition (chromium, nickel, titanium, silicon, manganese, etc.) and the internal structure, which plays a decisive role in the chromium element, it can form a passivation film on the surface of steel, the metal and the outside world isolation does not produce oxidation, enhance the corrosion resistance of steel plate. The number “8” in 8K refers to the proportion of the alloy content, and the letter “K” refers to the level of reflectivity achieved after polishing (K is the mirror reflection level). 8K mirror is the mirror grade of chromium-nickel alloy steel.

 

Common mirror stainless steel also includes 6K,10K,12K, etc., the larger the number, the finer the mirror is also higher. 6K refers to the rough grinding and polishing mirror plate, 10K refers to the fine grinding and polishing mirror panel, equivalent to the ordinary mirror; And 12K refers to the ultra-fine grinding polishing mirror panel, which can meet optical purposes. The higher the brightness, the greater the reflectivity and the less surface defects. In some non-strict singing they can be collectively referred to as 8K. The main polishing techniques used to obtain high-quality mirror stainless steel are electrolytic polishing, chemical polishing and mechanical polishing.

 

Electrolytic polishing

Electrolytic polishing is to soak in the electrolyte to obtain high-quality stainless steel on the surface of a polishing process, the stainless steel as an anode in this process, with the help of a direct current flows through the electrolyte specific solution to a metal, the anode surface to form a high resistivity of a thick mucous membrane, the thick mucous membrane in the micro concave and convex surface of stainless steel products in different thickness, Lead to the anode surface current density of the micro-distribution is not uniform, the current density in the bulge, dissolves fast, the concave current density is small, dissolve slowly, so as to reduce the surface roughness of stainless steel, improve the level and brightness, and form a passivation layer without defects. The electrolytic polishing solution must contain sufficient oxidizer and no active ions can destroy the passivation film.

 

Chemical polishing

The chemical polishing and electrolytic polishing principle are similar, the stainless steel is placed in a certain composition of the solution, the surface of the micro-raised part of the dissolution rate is greater than the micro-concave part of the dissolution rate, and the stainless steel surface is smooth, smooth. It can be seen that the principle of the chemical polishing method and electrolytic polishing method is basically the same, but the electrolytic polishing in the addition of voltage electrolysis under the forced action to accelerate the dissolution of the raised part, and the chemical polishing method is completely dependent on the self-corrosion ability of the solution to smooth the surface of stainless steel.

 

Mechanical polishing

Mechanical polishing refers to the high-speed rotating polishing wheel with polishing paste to mechanically eliminate the uneven surface of stainless steel and obtain a bright surface processing. The polishing wheel is used to distinguish its granularity level according to the different kinds of cloth made by it, and the main structure forms are sutured type, folding type, and so on. Polishing paste according to polishing needs by polishing ability of chromium oxide and binder composed of green polishing paste, there are also by abrasive, organic paste, additives composed of polishing wax. Mechanical polishing is generally divided into rough polishing, fine polishing, or at the same time polishing with different polishing paste and polishing wheel, under the action of mechanical rotation, the final reflection image of clear mirror stainless steel. When the user chooses BA stainless steel for mirror polishing operation, no rough polishing process is required.

Stainless steel pipe grades for oil and gas field

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Generally speaking, some low alloy steels can meet the requirements for corrosive oil and gas environment containing H2S, but the corrosive environment containing CO2 or H2S, CO2, Cl – coexistence where the Martensitic stainless steel need, duplex stainless steel or even nickel-based alloy. The 1988 version of API 5CT added corrosion-resistant tubing steel grades, specified the C75 steel grade with Martensitic stainless steel grades of 9Cr and 13Cr

 

High strength Martensitic stainless steel pipe for oil well

 In the wet environment with CO2 as the main gas, local corrosion damage of oil well pipe often occurs, such as pitting corrosion and intergranular corrosion, etc. If Cl – exists, the local corrosion will be intensified. It is generally considered that the corrosion can be ignored when the Carbon dioxide pressure is lower than 0.021MPa, and the corrosion will occur when the carbon dioxide pressure reaches 0.021MPa. When the pCO2 is higher than 0.021MPa, appropriate anti-corrosion measures should be taken. Generally, there is no damage caused by pitting when the co2 fraction is lower than 0.05Mpa.

It has been proved that the effect of using a sustained-release agent to prevent CO2 corrosion is limited, and the effect of using high chromium steel such as 9%-13%Cr steel is better. Since the 1970s, some natural gas Wells have used 9%Cr and 13Cr% stainless steel tubing to prevent CO2 corrosion. The American Petroleum Institute (API) recommends 9Cr and 13Cr martensitic stainless steel tubes (API L80-9Cr and L80-13Cr) for standardized use. 13Cr steel has better resistance to CO2 corrosion, while 9Cr-1Mo steel has better resistance to H2S stress corrosion cracking. In principle, neither steel is suitable if H2S is present in a CO2 atmosphere. When H2S exists in CO2 oil well, the SSCC resistance of the oil well pipe should be improved as far as possible, and the quenching and tempering heat treatment should be adopted to obtain uniform martensite and the hardness should be controlled below HRC22 as far as possible.

The stainless steel grade of oil well

Grade C Mo Cr Ni Cu
9Cr ≤0.15 0.9-1.1 8.0-10.0 ≤0.5 /
13Cr 0.15-0.22 / 12.0-14.0 ≤0.5 /
SUP9Cr ≤0.03 1.5-2.5 12.0-13.5 4.0-6.0 /
SUP13Cr ≤0.03 1.5-2.5 14.0-16.0 5.0-7.0 0.5-1.5

However, API 13Cr steel tubes have significantly reduced CO2 resistance and shortened service life when the oil well temperature reaches 150℃ or higher. In order to improve the CORROSION resistance of API 13Cr steel tubes to CO2 and SSC (sulfide stress cracking), low carbon SUP13Cr steel tubes with Ni and Mo added were developed. The steel tube can be used in wet environments with high temperatures, high CO2 concentrations and a small amount of hydrogen sulfide. The structure of these tubes is tempered martensite and less than 5% ferrite. The corrosion resistance to CO2 can be improved by reducing carbon or adding Cr and Ni, and the corrosion resistance to pitting can be improved by adding Mo. Compared with API 13Cr steel pipe, the corrosion resistance to CO2 and SSC is greatly improved. For example, in the same corrosive environment, the corrosion rate of API 13Cr steel pipe is more than 1mm/a, while the corrosion rate of SUP13Cr steel pipe is reduced to 0.125mm/a. With the development of deep and ultra-deep wells, the oil well temperature continues to increase. If the oil well temperature is further increased to more than 180℃, the corrosion resistance of SUP13Cr oil well pipe also begins to decline, which cannot meet the requirements of long-term use. According to the traditional material selection principle, duplex stainless steel or Nickel base alloy should be selected.

 

Martensitic stainless steel pipe for oil pipeline

The pipeline pipe conveying corrosive oil and gas requires the same corrosion-resistant material as the oil well pipe. Previously, the pipe was usually injected with sustained-release agents or corrosion-resistant materials such as dual-phase stainless steel. The former is unstable in anticorrosion effect at high temperature and may cause environmental pollution. Although dual-phase stainless steel has good corrosion resistance, the cost is high, and welding heat input is difficult to control, welding preheating and post-welding heat treatment to the construction of the site brings difficulties. The martensitic 11Cr pipe for CO2 environment and the martensitic 12Cr pipe for CO2+ trace H2S environment are put into use. The column has good weldability, without preheating and post-weld heat treatment, its mechanical properties can be equal to X80 steel grade, and its corrosion resistance is better than that of the pipeline with retarded release agent or dual-phase stainless steel pipe.

Stainless steel pipe for pipeline

Grade C Cr Ni Mo
11Cr ≤0.03 11 1.5 /
12Cr ≤0.03 12 5.0 2.0

 

Duplex stainless steel pipe for the petroleum industry

The martensitic stainless steel SUP 15Cr cannot meet the corrosion resistance requirements when the temperature of the oil (gas) well containing CO2 exceeds 200℃, and duplex stainless steel with good resistance to CO2 and Cl — stress corrosion cracks is required. Currently, 22Cr and 25Cr duplex (Austenitic and Ferrite) stainless steels are suitable for CO2 Wells above 200℃, while manufacturers adjust Cr and Ni content to adjust corrosion resistance. Duplex steel is composed of ferrite plus the Austenitic phase. Besides Cr and Ni, Mo and N can be added to improve the corrosion resistance. In addition to the duplex stainless steel has good high-temperature corrosion resistance, compared with martensite stainless steel, it has better H2S stress corrosion cracking resistance, at room temperature NACE TM 0177-A test, in A solution, 85%SMYS loading environment, martensite stainless steel can only pass the 10kPa H2S partial pressure test, Duplex stainless steel 25Cr can pass 100kPa H2S partial pressure test.

 

In general, in the coexistence of CO2 and H2S environments, or H2S partial pressure does not reach critical but Cl- is very high, 13Cr steel (including super 13Cr steel) can not meet the requirements, 22Cr duplex stainless steel (ASF 2205) or super duplex stainless steel 25Cr, Even high Ni, Cr stainless steel and Ni-based and Fe-Ni based alloys such as G3, alloy 825 containing more than 20% Cr, Ni30% are required.

How the alloying element affect the stainless steel?

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Chemical composition has a great influence on the microstructure, mechanical properties, physical properties and corrosion resistance of steel. Chromium, molybdenum, nickel and other alloying elements can replace the vertex Angle of the austenite lattice and the center of the six sides of the cube iron, carbon and nitrogen are located in the gap between the lattice atoms (gap position) due to small volume, produce huge strain in the lattice, so become effective hardening elements. Different alloying elements have different effects on the properties of steel, sometimes beneficial and sometimes harmful. The main alloying elements of Austenitic stainless steel have the following effects:

 

Cr

Chromium is an alloying element that makes stainless steel “rust free”. At least 10.5% chromium is required to form the surface passivation film characteristic of stainless steel. The passivation film can make stainless steel effectively resist corrosive water, a variety of acid solutions and even strong oxidation of high-temperature gas corrosion. When the chromium content exceeds 10.5%, the corrosion resistance of stainless steel is enhanced. The chromium content of 304 stainless steel is 18%, and some high-grade Austenitic stainless steels have chromium content as high as 20% to 28%.

 

Ni

Nickel can form and stabilize the Austenitic phase. 8%Ni makes 304 stainless steel, giving it the mechanical properties, strength and toughness required by austenite. High-performance austenitic stainless steels contain high concentrations of chromium and molybdenum, and nickel is added to maintain the austenitic structure when more chromium or other ferrite forming elements are added to the steel. The austenite structure can be guaranteed by about 20% nickel content, and the stress corrosion fracture resistance of stainless steel can be greatly improved.

Nickel can also reduce the work hardening rate during cold deformation, so the alloys used for deep drawing, spinning and cold heading generally have a high nickel content.

 

Mo

Molybdenum improves the pitting and crevice corrosion resistance of stainless steel in a chloride environment. The combination of molybdenum and chromium, especially nitrogen, makes the high-performance austenitic stainless steel have strong resistance to pitting and crevice corrosion. Mo can also improve the corrosion resistance of stainless steel in reductive environments such as hydrochloric acid and dilute sulfuric acid. The minimum molybdenum content of Austenitic stainless steel is about 2%, such as 316 stainless steel. High-performance Austenitic stainless steels with the highest alloy content contain up to 7.5% molybdenum. Molybdenum contributes to the formation of the Ferrite phase and affects the phase equilibrium. It is involved in the formation of several harmful secondary phases and will form unstable high-temperature oxides, have a negative impact on high-temperature oxidation resistance, the use of molybdenum-containing stainless steel must be taken into account.

 

C

Carbon stabilizes and strengthens the Austenitic phase. Carbon is a beneficial element for stainless steel used in high temperature environments such as boiler tubes, but in some cases can have a detrimental effect on corrosion resistance. The carbon content of most Austenitic stainless steel is usually limited to the lowest practicable level. The carbon content of welding grades (304L, 201L and 316L) is limited to 0.030%. The carbon content of some high alloy high-performance grades is even limited to 0.020%.

 

N

Nitrogen stabilizes and strengthens the Austenite phase, and slows down carbide sensitization and secondary phase formation. Both standard austenitic stainless steels and high performance austenitic stainless steels contain nitrogen. In low carbon grade (L), a small amount of nitrogen (up to 0.1%) can compensate for the loss of strength due to low carbon content. Nitrogen also helps improve resistance to chloride pitting and crevice corrosion, so some of the best corrosion-resistant high-performance austenitic stainless steels have nitrogen content as high as 0.5%.

 

Mn

Steel mills use manganese to deoxidize molten steel, so a small amount of manganese remains in all stainless steel. Manganese can also stabilize the Austenitic phase and improve the solubility of nitrogen in stainless steel. Therefore, in 200 series stainless steel, manganese can be used to replace part of the nickel to increase the nitrogen content, improve the strength and corrosion resistance. Manganese is added to some high-performance Austenitic stainless steels to achieve the same effect.

 

Cu

Copper can improve the corrosion resistance of stainless steel in reducing acids, such as some mixed solutions of sulfuric and phosphoric acid.

 

Si

In general, silicon is a beneficial element in Austenitic stainless steel because it can improve the corrosion resistance of steel in concentrated acid and a high oxidation environments. It is reported that UNS S30600 and other high silicon special stainless steels have high pitting corrosion resistance. Silicon, like manganese, can also be used to deoxidize molten steel, so small oxide inclusions containing silicon, manganese and other deoxidizing elements always remain in steel. But too many inclusions will affect the surface quality of the product.

 

Nb and Ti

These two elements are strong carbide-forming elements and can be used in place of low carbon grades to mitigate sensitization. Niobium carbide and titanium carbide can improve the high-temperature strength. 347 and 321 stainless steels containing Nb and Ti are commonly used in boilers and refining equipment to meet high temperature strength and weldability requirements. They are also used in some deoxidation processes as residual elements in high performance Austenitic stainless steels.

 

S and P

Sulfur is both good and bad for stainless steel. It can improve the machining performance, the harm is to reduce the thermal workability, increase the number of manganese sulfide inclusion, resulting in stainless steel pitting corrosion resistance reduced. High-grade Austenitic stainless steel is not easy to heat process, so the sulfur content should be controlled at the lowest level as far as possible, about 0.001%. Sulfur is not normally added as an alloying element to high-performance austenitic stainless steels. However, the sulfur content of standard grade stainless steel is often high (0.005% ~ 0.017%), in order to improve the weld penetration depth of self-fusion welding, improve cutting performance.

Phosphorus is a harmful element and can adversely affect the hot working properties of forging and hot rolling. In the cooling process after welding, it will also promote the occurrence of thermal cracking. Therefore, phosphorus content should be controlled at a minimum level.

Why dental instruments are made of stainless steel?

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Many types of tools are used to clean and care for teeth, including probes, mirrors, scrapers, dental burnishers and pressors. Mirrors help examine the patient’s mouth, and scrapers scrape to remove plaque and tartar. The polisher gives a final finish to the fill, smoothing out scratches left by other tools. The probe is used to find the cavity and pressure area of the tooth so that the restorative material can be placed. They have a variety of angles and pointed shapes, so the dentist can freely reach all sides of the teeth. A variety of materials are available to manufacture dental instruments, including stainless steel, carbon steel, titanium, and plastics. Important factors to consider when choosing a tool include strength and toughness of the material, weight, balance, ability to maintain sharp edges, and corrosion resistance.

Dental instruments should have enough strength and toughness to prevent their fracture and avoid stabbing accidents. Stainless steel offers the most suitable properties for each class of instrument. The high hardness of surgical stainless steel maximizes tip life and reduces maintenance time. Stainless steel tips have excellent toughness, scrapers and probes require sharp edges to reduce the pressure applied by the dentist, thus avoiding damage to the patient’s teeth or the tool itself. Blunt instruments are difficult to use, reducing the quality and accuracy of the operation and taking up more time for dentists.

As with all medical practices, cleanliness is a key factor to the safety and success of dental practices. Dental appliances need to be disinfected after each use, usually by means of high-temperature steam disinfection in an autoclave using dry heat sterilization or chemical steam pressure sterilization. Stainless steel is resistant to corrosion during any of these sterilized treatments and its inert surfaces are easily cleaned and disinfected. Scrapers are used to remove hardened dental plaque from the surface of teeth.

A widely used grade is AISI 440A, a high-carbon, 0.75% molybdenum hardened stainless steel. A manufacturer in California uses the Model 440A to manufacture high-quality dental and surgical instruments. According to the experience of the company’s metallurgists, this grade offers the best hardness, toughness and wear resistance of any stainless steel. Another top tool manufacturer in the United States uses 440A stainless steel to make durable, reliable, and high-quality instruments that enable dentists and technicians to achieve the best in medical practice and patient care.

A German dental instrument manufacturer manufactures probes using super duplex stainless steel containing 3% molybdenum. The super duplex stainless steel has high strength, good toughness and excellent wear resistance, ensuring that the tip of the instrument remains sharp for a long time. Sandvik, a stainless steel manufacturer, has offered a range of molybdenum-containing grades for medical and dental instruments – molybdenum-containing 4% precipitation hardening (PH) grade. It can be formed at low hardness, then heat-treated to reach final hardness in one step, and has better toughness than the hardened martensite grade, which requires more heat treatment steps.