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Solution annealing treatments are applied to a series of alloy steel and stainless steel compositions. Solution annealing of 300 series stainless steel castings produces a uniform microstructure free of carbides in the microstructure. Solution annealing of precipitation hardening cast and wrought alloys produces a soft microstructure suitable for machining to close tolerances. These alloys have the potential for Age Hardening after machining with minimal distortion of close dimensional tolerances. These materials and processes are generally applied to medium strength requirements on turned or screw machine parts. The heat treatments may be performed in batch atmosphere furnaces, non-atmosphere furnaces, and vacuum furnaces, depending on the part size, geometry, and surface condition required. Small parts may also be treated in continuous hydrogen belt furnaces. Solution annealing and age hardening may also be applied to aluminum alloy stampings and castings. These treatments are usually performed in non-atmosphere batch furnaces with water quenching of the parts after solution annealing, and age hardening in electric or gas fired batch furnaces in air.

Austenitic stainless steels have high ductility, low yield stress and relatively high ultimate tensile strength, when compare to a typical carbon steel.

A carbon steel on cooling transforms from Austenite to a mixture of ferrite and cementite. With austenitic stainless steel, the high chrome and nickel content suppress this transformation keeping the material fully austenite on cooling (The Nickel maintains the austenite phase on cooling and the Chrome slows the transformation down so that a fully austenitic structure can be achieved with only 8% Nickel). 

Heat treatment and the thermal cycle caused by welding, have little influence on mechanical properties.  However strength and hardness can be increased by cold working, which will also reduce ductility.  A full solution anneal (heating to around 1045°C followed by quenching or rapid cooling) will restore the material to its original condition, removing alloy segregation, sensitisation, sigma phase and restoring ductility after cold working.  Unfortunately the rapid cooling will re-introduce residual stresses, which could be as high as the yield point.  Distortion can also occur if the object is not properly supported during the annealing process.

Austenitic steels are not susceptible to hydrogen cracking, therefore pre-heating is seldom required, except to reduce the risk of shrinkage stresses in thick sections.  Post weld heat treatment is seldom required as this material as a high resistance to brittle fracture; occasionally stress relief is carried out to reduce the risk of stress corrosion cracking, however this is likely to cause sensitisation unless a stabilised grade is used  (limited stress relief can be achieved with a low temperature of around 450°C ). 

Austenitic steels have a F.C.C atomic structure which provides more planes for the flow of dislocations, combined with the low level of interstitial elements (elements that lock the dislocation chain), gives this material its good ductility. This also explains why this material has no clearly defined yield point, which is why its yield stress is always expressed as a proof stress. Austenitic steels have excellent toughness down to true absolute (-273°C), with no steep ductile to brittle transition. 

This material has good corrosion resistance, but quite severe corrosion can occur in certain environments. The right choice of welding consumable and welding technique can be crucial as the weld metal can corrode more than the parent material.

Probably the biggest cause of failure in pressure plant made of stainless steel is stress corrosion cracking (S.C.C).  This type of corrosion forms deep cracks in the material and is caused by the presence of chlorides in the process fluid or heating water/steam (Good water treatment is essential ), at a temperature above 50°C, when the material is subjected to a tensile stress (this stress includes residual stress, which could be up to yield point in magnitude). Significant increases in Nickel and also Molybdenum will reduce the risk. 

Stainless steel has a very thin and stable oxide film rich in chrome. This film reforms rapidly by reaction with the atmosphere if damaged.  If stainless steel is not adequately protected from the atmosphere during welding or is subject to very heavy grinding operations, a very thick oxide layer will form. This thick oxide layer, distinguished by its blue tint, will have a chrome depleted layer under it, which will impair corrosion resistance.  Both the oxide film and depleted layer must be removed, either mechanically (grinding with a fine grit is recommended, wire brushing and shot blasting will have less effect), or chemically (acid pickle with a mixture of nitric and hydrofluoric acid).  Once cleaned, the surface can be chemically passivated to enhance corrosion resistance, (passivation reduces the anodic reaction involved in the corrosion process).

Carbon steel tools, also supports or even sparks from grinding carbon steel, can embed fragments into the surface of the stainless steel.  These fragments can then rust if moistened. Therefore it is recommended that stainless steel fabrication be carried out in a separate designated area and special stainless steel tools used where possible.

 If any part of stainless-steel is heated in the range 500 degrees to 800 degrees for any reasonable time there is a risk that the chrome will form chrome carbides (a compound formed with carbon) with any carbon present in the steel.  This reduces the chrome available to provide the passive film and leads to preferential corrosion, which can be severe. This is often referred to as sensitisation.  Therefore it is advisable when welding stainless steel to use low heat input and restrict the maximum interpass temperature to around 175°, although sensitisation of modern low carbon grades is unlikely unless heated for prolonged periods.  Small quantities of either titanium (321) or niobium (347) added to stabilise the material will inhibit the formation of chrome carbides.

To resist oxidation and creep high carbon grades such as 304H or 316H are often used.  Their improved creep resistance relates to the presence of carbides and the slightly coarser grain size associated with higher annealing  temperatures.  Because the higher carbon content inevitably leads to sensitisation, there may be a risk of corrosion during plant shut downs, for this reason stabilised grades may be preferred such as 347H.

The solidification strength of austenitic stainless steel can be seriously impaired by small additions of impurities such as sulphur and phosphorous, this coupled with the materials high coefficient of expansion can cause serious solidification cracking problems.  Most 304 type alloys are designed to solidify initially as delta ferrite, which has a high solubility for sulphur, transforming to austenite upon further cooling. This creates an austenitic material containing tiny patches of residual delta ferrite, therefore not a true austenitic in the strict sense of the word.  Filler metal often contains further additions of delta ferrite to ensure crack free welds.

The delta ferrite can transform to a very brittle phase called sigma, if heated above 550°C for very prolonged periods  (Could take several thousand hours, depending on chrome level.  A duplex stainless steel can form sigma phase after only a few minutes at this temperature)

The very high coefficient of expansion associated with this material means that welding distortion can be quite savage.  I have seen thick ring flanges on pressure vessel twist after welding to such an extent that a fluid seal is impossible.  Thermal stress is another major problem associated with stainless steel; premature failure can occur on pressure plant heated by a jacket or coils attached to a cold veesel.  This material has poor thermal conductivity, therefore lower welding current is required (typically 25% less than carbon steel) and narrower joint preparations can be tolerated.  All common welding processes can be used successfully, however high deposition rates associated with SAW could cause solidification cracking and possibly sensitisation, unless adequate precautions are taken. 

To ensure good corrosion resistance of the weld root it must be protected from the atmosphere by an inert gas shield during welding and subsequent cooling.  The gas shield should be contained around the root of the weld by a suitable dam, which must permit a continuous gas flow through the area.  Welding should not commence until sufficient time has elapsed to allow the volume of purging gas flowing through the dam to equal at least the 6 times the volume contained in the dam (EN1011 Part 3 Recommends 10).   Once purging is complete the purge flow rate should be reduced so that it only exerts a small positive pressure, sufficient to exclude air.  If good corrosion resistance of the root is required the oxygen level in the dam should not exceed 0.1%(1000 ppm); for extreme corrosion resistance this should be reduced to 0.015% (150 ppm).  Backing gasses are typically argon or helium; Nitrogen Is often used as an economic alternative where corrosion resistance is not critical, Nitrogrn + 10% Helium is better.  A wide variety of proprietary pastes and backing materials are available than can be use to protect the root instead of a gas shield.  In some applications where corrosion and oxide coking of the weld root is not important, such as large stainless steel ducting, no gas backing is used.

Carbon content:
304 L grade  Low Carbon, typically    0.03% Max
304   grade   Medium Carbon, typically   0.08% Max
304H grade  High Carbon, typically  Up to 0.1% 

The higher the carbon content the greater the yield strength.  (Hence the stength advantage in using stabilised grades)

Investment casting, often called lost wax process casting, is regarded as a precision casting process to fabricate near net-shaped metal parts from almost any alloy and is typically used for the production of components requiring complex, often thin-wall castings.

There is a long and rich history associated with investment casting, dating back thousands of years to the production of bronze, copper and gold jewelry, idols and statues as far back as the ancient Egypt and Mesopotamia, the Han Dynasty in China, the Aztecs in Mexico and the Benin civilization in Africa. The earliest known text describing the investment casting process was written by the monk Theophilius Presbyter around 1100 A.D. His writings were used by sculptor and goldsmith Benvenuto Cellini (1500 – 1571), as described in his autobiography, for the production of the Perseus and Head of Medusa sculpture that still stands today in Loggia dei Lanzi in Florence, Italy. In fact, by the mid 1500′s the investment casting technique was considered “ordinary procedure” for making bronze statues and other works of art.

The technique reemerged in the late 19th century when dentists began using the technique to make crowns and inlays, following the publication of a paper by Dr. D. Philbrook of Council Bluffs, Iowa in 1897. It was Dr. William H. Taggers of Chicago, however, who spearheaded the use and growth of investment casting as a modern industrial process, following publication of a paper in 1907 that detailed the development of a technique that utilized a wax pattern compound of excellent properties, the development of an investment material and the invention of an air-pressure casting machine.

Use of investment casting accelerated significantly during the 1940′s as a result of military demands on the machine tool industry. Investment casting proved to be an efficient, reliable and cost-effective method for meeting military demands for near net-shape precision parts and use of specialized alloys that could not be shaped by traditional methods, or required too much machining. Following the war, the technique expanded into many commercial and industrial applications that used complex metal parts.  It was during this time that Bimac emerged and ultimately evolved into one of the leading investment casting providers in the Midwest.

The major advantages of investment casting are extreme versatility, cost effectiveness and the precise dimension control it offers. It saves time and labor and ensures error-free, accurate dimensional parts, giving designers and engineers more flexibility.

Versitility

The versatility is the major advantage of investment casting. It supports the production of a wide range of products, for a variety of industries. The fabrication of some designs can be difficult or impossible with other metal casting methods. Investment casting allows the production of thin-wall, finely detailed parts, with extreme precision control. It also allows the production of quality parts that are both ultra light and strong, with extremely accurate details. This technique avoids the necessity of welding and joining together different parts.

Investment casting can be used with a wide variety of metals and alloys. This gives designers and engineers the freedom to concentrate on functional aspect of the casting rather than on its production. This is also a major advantage of investment casting.

Accuracy For Even the Finest Details

Not only does it give a lightweight option for high precision parts, but the precise dimensional control also produces a casting that requires little to no machining or final detailing. It comes with the best surface finish, which is not possible by sand casting or other methods of metal casting. It also save on cost and time as it needs no welding, assembling or finishing work. It avoids the need to produce multiple parts and then weld them together as a unit. This technique produces castings with thin walls, with accuracy for even the finest details.

Cost Effectiveness

Investment casting cuts cost on finishing, machining and grinding. It also combines with itself for complex sub-assembly parts. This reduces costly welding and weak joints and welds. Major automobile companies and even jet engine manufacturers now rely on investment casting to produce lightweight precise dimension parts.

Precision Dimension Control

In jewelry making, high precision dimen-sion control is necessary to produce the desired designs with fine details. This is one advantage of investment casting. Parts that require the sharpest finish can be cast without extra effort. It is only necessary to create or design a model from which an exact replica can be produced using aluminum alloy casting.

The rapid solidification process when used in investment casting can unite both high strength and thin wall capabilities. OEM companies, fully aware of the advan-tages of investment casting, now produce high-end airframe components and jet engine parts using this technology.

Saving Costs, Time and Labor

The development of different alloys equipped companies to manufacture complex parts, which were previously a combination of several parts. This has ensured high levels of mechanical properties while saving costs, time, and labor. It also reduces the chances of error in the form of dimensional variations that can occur when combining different parts together. Designers and engineers are now free to concentrate on the application of parts rather than on their production.

Investment casting got its name from the technique utilized by the Egyptians over 4,000 years ago to produce jewelry. Since gold is an investment, casting gold can be considered investment casting. Investment casting is used to produce jewelry and high-precision parts. Investment casting aluminum structures can have a wall dimension of less than a millimeter (0.40 to 0.75 mm) or up to several inches.

Investment casting can produce items in sizes and shapes that cannot be made by other metal casting methods. During the Second World War a huge demand for investment casting developed to furnish the jet propulsion systems in airplanes. Investment casting of mechanical parts later found its way to civilian aircraft.

Can Produce High-Precision Mechanical Parts

Since the invention of the jet engine, there has been increased interest in investment casting by the aerospace industry. It is reliable for its ability to produce accurate dimensional, high-precision casting parts. Increased resistance to corrosion and lower maintenance costs are the hallmarks of aluminum alloy investment casting. Investment casting can produce high-precision mechanical and engineering parts quickly and in large quantities.

Can Produce High-Precision Mechanical Parts

The process of investment casting is very simple. Plaster slurry is applied to a design formed from a fusible substance like wax. As the plaster bakes, the wax melts out. The name “lost-wax casting” is a perfect description of the process. The internal cavity thus formed is the shell or mould for the aluminum investment casting. The molten aluminum is poured into the cavity. After cooling, the external mold is broken and the casting is retained.

Offers the Greatest Design Flexibility

Investment casting has many advantages. It is a very versatile process and produces a wide range of products for a variety of industries. It allows the selection of suitable alloys, such as aluminum alloys. Investment casting offers the greatest design flexibility for producing complex structures. This flexibility cannot be found in other metal casting methods. This allows the engineers and designers to concentrate on the functional aspect of the part rather than its manufacture.

Precise Dimensional Control

Investment casting gives a lightweight option for precision parts. The precise dimensional control also allows the casting to be used with little or no machining for final detailing. It produces a surface finish that cannot be obtained by machining or sanding. It also saves on cost and time, as it needs no welding, assembling or finishing work. It also saves on the need to produce multiple parts and to weld them together as a unit.

It is connected with the ancient Egyptian method of creating gold jewelry. It can produce gold jewelry with extremely minute details and can also produce precision parts that are just a few grams (i.e. 5g) in weight.

Deliver What Their Customer Demands

Big manufacturing companies outsource the production of precision parts. The OEM companies are able to deliver what their customer demands, mostly due to the precision possible by investment casting. This is the same for a number of industries including the manufacturers of automobiles or missiles.

Certifications are available for different aluminum investment castings. The quality is determined based on the strength and tensile ductility for the casting. The porosity is a negative quality and affects the internal strength.

Molybdenum is used efficiently and economically in alloy steel & iron to

–improve hardenability

–reduce temper embrittlement

–resist hydrogen attack & sulphide stress cracking

–increase elevated temperature strength

–improve weldability, especially in high strength low alloy steels (HSLA) .

In the present section the focus is on grades and properties of Mo containing alloy steel and iron. End uses cover the whole world of engineered products for:

-Automotive, shipbuilding, aircraft and aerospace

-Drilling, mining, processing

-Energy generation, including boilers, steam turbines and electricity generators

-Vessels, tanks, heat exchangers

-Chemical & Petrochemical processing Offshore;

-Oil Country Tubular Goods (OCTG)

In most cases molybdenum is needed to meet the high end of the application properties, which is accomplished with comparatively small molybdenum additions. In fact, with the exception of High Speed Steel and Maraging Steel the Mo content often ranges between 0.2 and 0.5% and rarely exceeds 1%.

Specifications define a couple hundred stainless steel grades, differing from one another mainly in chemical composition. Composition is a fundamental characteristic of stainless steel because it determines the alloy’s corrosion resistance, microstructural phase balance, mechanical properties, and physical properties. The table below lists the most important stainless steel grades and the most important molybdenum-containing grades. The most widely used grades are austenitic Types 304 and 316 and ferritic Types 409 and 430.

Table 1:  Chemican composition and Pitting Resistance Equivalent Number (PREN) of common ferritic, austenitic and duplex stainless steels with and without molybdenum.

The Pitting Resistance Equivalent Number (PREN) is a measure of the relative pitting corrosion resistance of stainless steel in a chloride-containing environment.  Higher PREN values indicate greater corrosion resistance.  The formula for PREN is:

PREN = %Cr + 3.3*%Mo + 16*%N

This formula suggests that molybdenum is 3.3 times more effective than chromium at improving pitting resistance, which is true within limits.  Chromium must always be present in stainless steel to provide basic corrosion resistance.  Molybdenum cannot provide this basic resistance, but it significantly enhances a stainless steel’s corrosion resistance, as the formula shows.

The table shows that ferritic, austenitic and duplex stainless steels with different levels of pitting resistance are available, and the figure below shows that it is possible to select several different grades with similar pitting resistance.  For example Types 444 (ferritic), 316 (austenitic) and 2304 (duplex) have similar resistance to pitting corrosion in chloride environments.

Selection of the appropriate stainless steel grade depends primarily on the corrosiveness of the application environment and the application’s mechanical property requirements. When more than one alloy meets these requirements, other factors like physical properties, fabricability, availability and cost are considerations.

Duplex stainless steels are called “duplex” because they have a two-phase microstructure consisting of grains of ferritic and austenitic stainless steel. The picture shows the yellow austenitic phase as “islands” surrounded by the blue ferritic phase. When duplex stainless steel is melted it solidifies from the liquid phase to a completely ferritic structure. As the material cools to room temperature, about half of the ferritic grains transform to austenitic grains (“islands”). The result is a microstructure of roughly 50% austenite and 50% ferrite.

The duplex structure gives this family of stainless steels a combination of attractive properties:

Strength: Duplex stainless steels are about twice as strong as regular austenitic or ferritic stainless steels.

Toughness and ductility: Duplex stainless steels have significantly better toughness and ductility than ferritic grades; however, they do not reach the excellent values of austenitic grades.

Corrosion resistance: As with all stainless steels, corrosion resistance depends mostly on the composition of the stainless steel. For chloride pitting and crevice corrosion resistance, their chromium, molybdenum and nitrogen content are most important. Duplex stainless steel grades have a range of corrosion resistance, similar to the range for austenitic stainless steels, i.e from Type 304 or 316 (e.g. LDX 2101©) to 6% molybdenum (e.g. SAF 2507©) stainless steels.

Stress corrosion cracking resistance: Duplex stainless steels show very good stress corrosion cracking (SCC) resistance, a property they have “inherited” from the ferritic side. SCC can be a problem under certain circumstances (chlorides, humidity, elevated temperature) for standard austenitics such as Types 304 and 316.

Cost: Duplex stainless steels have lower nickel and molybdenum contents than their austenitic counterparts of similar corrosion resistance. Due to the lower alloying content, duplex stainless steels can be lower in cost, especially in times of high alloy surcharges. Additionally, it may often be possible to reduce the section thickness of duplex stainless steel, due to its increased yield strength compared to austenitic stainless steel. The combination can lead to significant cost and weight savings compared to a solution in austenitic stainless steels.

One of the features that characterize stainless steels is a minimum 10.5% chromium content as the principal alloying element. Four major categories of wrought stainless steel, based on metallurgical structure, are austenitic, ferritic, martensitic, and precipitation hardening. Cast stainless-steel grades are generally designated as either heat resistant or corrosion resistant.

Austenitic wrought stainless steel are classified in three groups:

• The AISI 200 series (alloys of iron-chromium-nickel-manganese)
• The AISI 300 series (alloys of iron-chromium-nickel)
• Nitrogen-strengthened alloys

Carbon content is usually low (0.15% or less), and the alloys contain a minimum of 16% chromium with sufficient nickel and manganese to provide an austenitic structure at all temperatures from the cryogenic region to the melting point of the alloy.

Nitrogen-strengthened austenitic stainless steels are alloys of chromium-manganese-nitrogen; some grades also contain nickel. Yield strengths of these alloys (annealed) are typically 50% higher than those of the nonnitrogen-bearing grades. They are nonmagnetic and most remain so, even after severe cold working.

Like carbon, nitrogen increases the strength of a steel. But unlike carbon, nitrogen does not combine significantly with chromium in a stainless steel. This combination, which forms chromium carbide, reduces the strength and corrosion resistance of an alloy.

Until recently, metallurgists had difficulty adding controlled amounts of nitrogen to an alloy. The development of the argon-oxygen decarburization (AOD) method has made possible strength levels formerly unattainable in conventional annealed stainless alloys.

Austenitic stainless steels are generally used where corrosion resistance and toughness are primary requirements. Typical applications include shafts, pumps, fasteners, and piping in seawater and equipment for processing chemicals, food, and dairy products.

Ferritic wrought alloys (the AISI 400 series) contain from 10.5 to 27% chromium. In addition, the use of argon-oxygen decarburization and vacuum-induction melting has produced several new ferritic grades including 18Cr-2Mo, 26Cr-1Mo, 29Cr-4Mo, and 29Cr-4Mo-2Ni. Low in carbon content, but generally higher in chromium than the martensitic grades, these steels cannot be hardened by heat treating and are only moderately hardened by cold working. Ferritic stainless steels are magnetic and retain their basic microstructure up to the melting point if sufficient Cr and Mo are present. In the annealed condition, strength of these grades is approximately 50% higher than that of carbon steels.

Ferritic stainless steels are typically used where moderate corrosion resistance is required and where toughness is not a major need. They are also used where chloride stress-corrosion cracking may be a problem because they have high resistance to this type of corrosion failure. In heavy sections, achieving sufficient toughness is difficult with the higher-alloyed ferritic grades. Typical applications include automotive trim and exhaust systems and heat-transfer equipment for the chemical and petrochemical industries.

Martensitic steels are also in the AISI 400 series. These wrought, higher-carbon steels contain from 11.5 to 18% chromium and may have small quantities of additional alloying elements. They are magnetic, can be hardened by heat treatment, and have high strength and moderate toughness in the hardened-and-tempered condition. Forming should be done in the annealed condition. Martensitic stainless steels are less resistant to corrosion than the austenitic or ferritic grades. Two types of martensitic steels — 416 and 420F — have been developed specifically for good machinability.

Martensitic stainless steels are used where strength and/or hardness are of primary concern and where the environment is relatively mild from a corrosive standpoint. These alloys are typically used for bearings, molds, cutlery, medical instruments, aircraft structural parts, and turbine components. Type 420 is used increasingly for molds for plastics and for industrial components requiring hardness and corrosion resistance.

Precipitation-hardening stainless steels develop very high strength through a low-temperature heat treatment that does not significantly distort precision parts. Compositions of most precipitation-hardening stainless steels are balanced to produce hardening by an aging treatment that precipitates hard, intermetallic compounds and simultaneously tempers the martensite. The beginning microstructure of PH alloys is austenite or martensite. The austenitic alloys must be thermally treated to transform austenite to martensite before precipitation hardening can be accomplished.

These alloys are used where high strength, moderate corrosion resistance, and good fabricability are required. Typical applications include shafting, high-pressure pumps, aircraft components, high-temper springs, and fasteners.

Cast stainless steels usually have corresponding wrought grades that have similar compositions and properties. However, there are small but important differences in composition between cast and wrought grades. Stainless-steel castings should be specified by the designations established by the ACI (Alloy Casting Institute), and not by the designation of similar wrought alloys.

Service temperature provides the basis for a distinction between heat-resistant and corrosion-resistant cast grades. The C series of ACI grades designates the corrosion-resistant steels; the H series designates the heat-resistant steels, which can be used for structural applications at service temperatures between 1,200 and 2,200°F. Carbon and nickel contents of the H-series alloys are considerably higher than those of the C series. H-series steels are not immune to corrosion, but they corrode slowly — even when exposed to fuel-combustion products or atmospheres prepared for carburizing and nitriding. C-series grades are used in valve, pumps, and fittings. H-series grades are used for furnace parts and turbine components.

Galling and wear are failure modes that require special attention with stainless steels because these materials serve in many harsh environments. They often operate, for example, at high temperatures, in food-contact applications, and where access is limited. Such restrictions prevent the use of lubricants, leading to metal-to-metal contact — a condition that promotes galling and accelerated wear.

In a sliding-wear situation, a galling failure mode occurs first, followed by dimensional loss due to wear, which is, in turn, usually followed by corrosion. Galling is a severe form of adhesive wear that shows up as torn areas of the metal surface. Galling can be minimized by decreasing contact stresses or by the use of protective surface layers such as lubricants (where acceptable), weld overlays, platings, and nitrided or carburized surface treatments.

Test results from stainless-steel couples (table) indicate the relatively poor galling resistance of austenitic grades and even alloy 17-4 PH, despite its high hardness. Among the standard grades, only AISI 416 and 440C performed well. Good to excellent galling resistance was demonstrated by Armco’s Nitronic 32 and 60 alloys (the latter were developed specifically for antigalling service).

Recent research findings prove that adding silicon to a high-manganese, nitrogen-strengthened austenitic stainless alloy produces a wear-resistant stainless steel. Wear and corrosion resistance are still considered unavoidable trade-offs in stainless, but the new formula promises to resist both conditions.

Beating corrosion is the number one reason for choosing stainless. But in cases where parts are difficult to lubricate, most stainless steels cannot resist wear. Under high loads and insufficient lubrication, stainless often sports a type of surface damage known as galling. In critical parts, galling can lead to seizure or freezing, which can shut down machinery.

Designers typically get around galling by using cast alloys or by applying a cobalt facing to stainless parts. Either way, the fixes can be expensive and may pose new problems that accompany the hard-facing process. These include maintaining uniform facing thickness and ensuring proper adhesion between facing and substrate. A new stainless formula aims to sidestep these difficulties by offering an alternative to expensive wear-resistant materials.

In search of a cost-effective alternative, researchers at Carpenter Technology, Reading, Pa., looked at elemental effects of silicon, manganese, and nickel on galling resistance of nitrogen-strengthened, austenitic stainless steels. Results of an initial test program determined that silicon was a catalyst for galling resistance, while nickel and manganese were not.

The silicon levels in a recently developed gall-resistant stainless alloy are between 3 and 4%. Silicon levels must remain lower than 5% to maintain the proper metallurgical structure. In addition, too much silicon decreases nitrogen solubility. To maintain strength, higher amounts of costly nickel would need to be added.

Researchers can now define optimum composition limits for a gall-resistant stainless steel. To prove the new steel’s validity, properties such as galling, wear, and corrosion are evaluated and compared with commercially available stainless steels. Four alloys, a gall-resistant austenitic alloy called Gall-Tough, another austenitic alloys with higher nickel and manganese content (16Cr-8Ni-4Si-8Mn), and Types 304 and 430 stainless steels are included in the comparison.

Results show the galling threshold for gall-resistant stainless is over 15 times higher than that of conventional stainless steels. In addition, gall-resistant stainless withstands more than twice the stress without galling compared to the 16Cr-8Ni-4Si-8Mn alloy. Yet, the new formula sacrifices only a slight amount of corrosion resistance.

For strength and hardness, both gall-resistant stainless and the 16Cr-8Ni-4Si-8Mn alloy beat Types 304 and 430 alloys. The new alloy also shows a uniquely high ultimate tensile strength, possibly due to martensite formation during tensile testing. Ductility for all four alloys is excellent. These findings indicate that gall-resistant alloys can economically bridge the gap between corrosion, galling, and metal-to-metal wear resistance.

During a project on the development of sensors for the powder metallurgy industry, NIST has developed a technique for production of nitrogenated stainless steel alloys with enhanced corrosion and mechanical properties. Discussions with metal powder producers are underway to develop commercial powder metallurgy alloys using the NIST process that will find applications in biomedical implant devices, light-weight armor plate, and other demanding environments. NIST developed this technique through work on a model for prediction of nitrogen solubility and microstructure in modified 300 series stainless steel alloys.

The new powder metallurgy nitrogenated stainless steel alloys are single phase (austenite) with no tendency to form the embrittling nitride and sigma phase compounds often found in high nitrogen stainless steels. The unique microstructure results in consolidated parts with superior corrosion and mechanical properties compared to commercially available wrought alloys, and reduced costs compared to other powder metallurgy nitrogenated stainless steel alloys.

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