Steels are among the most commonly used alloys. The complexity of steel alloys is fairly significant. Not all effects of the varying elements are included. The following text gives an overview of some of the effects of various alloying elements. Additional research should be performed prior to making any design or engineering conclusions.
Carbon has a major effect on steel properties. Carbon is the primary hardening element in steel. Hardness and tensile strength increases as carbon content increases up to about 0.85% C as shown in the figure above. Ductility and weldability decrease with increasing carbon.
Manganese is generally beneficial to surface quality especially in resulfurized steels. Manganese contributes to strength and hardness, but less than carbon. The increase in strength is dependent upon the carbon content. Increasing the manganese content decreases ductility and weldability, but less than carbon. Manganese has a significant effect on the hardenability of steel.
Phosphorus increases strength and hardness and decreases ductility and notch impact toughness of steel. The adverse effects on ductility and toughness are greater in quenched and tempered higher-carbon steels. Phosphorous levels are normally controlled to low levels. Higher phosphorus is specified in low-carbon free-machining steels to improve machinability.
Sulfur decreases ductility and notch impact toughness especially in the transverse direction. Weldability decreases with increasing sulfur content. Sulfur is found primarily in the form of sulfide inclusions. Sulfur levels are normally controlled to low levels. The only exception is free-machining steels, where sulfur is added to improve machinability.
Silicon is one of the principal deoxidizers used in steelmaking. Silicon is less effective than manganese in increasing as-rolled strength and hardness. In low-carbon steels, silicon is generally detrimental to surface quality.
Copper in significant amounts is detrimental to hot-working steels. Copper negatively affects forge welding, but does not seriously affect arc or oxyacetylene welding. Copper can be detrimental to surface quality. Copper is beneficial to atmospheric corrosion resistance when present in amounts exceeding 0.20%. Weathering steels are sold having greater than 0.20% Copper.
Lead is virtually insoluble in liquid or solid steel. However, lead is sometimes added to carbon and alloy steels by means of mechanical dispersion during pouring to improve the machinability.
Boron is added to fully killed steel to improve hardenability. Boron-treated steels are produced to a range of 0.0005 to 0.003%. Whenever boron is substituted in part for other alloys, it should be done only with hardenability in mind because the lowered alloy content may be harmful for some applications.
Boron is a potent alloying element in steel. A very small amount of boron (about 0.001%) has a strong effect on hardenability. Boron steels are generally produced within a range of 0.0005 to 0.003%. Boron is most effective in lower carbon steels.
Chromium is commonly added to steel to increase corrosion resistance and oxidation resistance, to increase hardenability, or to improve high-temperature strength. As a hardening element, Chromium is frequently used with a toughening element such as nickel to produce superior mechanical properties. At higher temperatures, chromium contributes increased strength. Chromium is a strong carbide former. Complex chromium-iron carbides go into solution in austenite slowly; therefore, sufficient heating time must be allowed for prior to quenching.
Nickel is a ferrite strengthener. Nickel does not form carbides in steel. It remains in solution in ferrite, strengthening and toughening the ferrite phase. Nickel increases the hardenability and impact strength of steels.
Molybdenum increases the hardenability of steel. Molybdenum may produce secondary hardening during the tempering of quenched steels. It enhances the creep strength of low-alloy steels at elevated temperatures.
Aluminum is widely used as a deoxidizer. Aluminum can control austenite grain growth in reheated steels and is therefore added to control grain size. Aluminum is the most effective alloy in controlling grain growth prior to quenching. Titanium, zirconium, and vanadium are also valuable grain growth inhibitors, but there carbides are difficult to dissolve into solution in austenite.
Zirconium can be added to killed high-strength low-alloy steels to achieve improvements in inclusion characteristics. Zirconium causes sulfide inclusions to be globular rather than elongated thus improving toughness and ductility in transverse bending.
Niobium (Columbium) increases the yield strength and, to a lesser degree, the tensile strength of carbon steel. The addition of small amounts of Niobium can significantly increase the yield strength of steels. Niobium can also have a moderate precipitation strengthening effect. Its main contributions are to form precipitates above the transformation temperature, and to retard the recrystallization of austenite, thus promoting a fine-grain microstructure having improved strength and toughness.
Titanium is used to retard grain growth and thus improve toughness. Titanium is also used to achieve improvements in inclusion characteristics. Titanium causes sulfide inclusions to be globular rather than elongated thus improving toughness and ductility in transverse bending.
Vanadium increases the yield strength and the tensile strength of carbon steel. The addition of small amounts of Vanadium can significantly increase the strength of steels. Vanadium is one of the primary contributors to precipitation strengthening in microalloyed steels. When thermomechanical processing is properly controlled the ferrite grain size is refined and there is a corresponding increase in toughness. The impact transition temperature also increases when vanadium is added.
All microalloy steels contain small concentrations of one or more strong carbide and nitride forming elements. Vanadium, niobium, and titanium combine preferentially with carbon and/or nitrogen to form a fine dispersion of precipitated particles in the steel matrix.
Effect of P & S on Hydrogen absorption of high strength steel
by Arindam | 1:44 PM in materials | comments (0)
Phosphorus and Sulphur present in high strength steel increases its susceptibility to corrosion cracking not only because of their embrittling effect but also because of hydrogen absorption.
Wrought stainless steels are solution annealed after processing and hot worked in order to dissolve carbides and sigma. Carbides may form during heating in the 425 to 900C (800 to 1650F) range or during slow cooling through this range. Sigma tends to form at temperatures below 925C (1700F). Specifications normally require solution annealing to be done at 1035C (1900F) with a rapid quench. The molybdenum-containing grades are frequently solution annealed at somewhat higher temperatures in the 1095 to 1120C (2000 to 2050F) in order to better homogenize the molybdenum.
Stainless steels may be stress relieved. There are several stress relief treatments. Guidelines follow.
Stress redistribution at 290 to 425C (550 to 800F), which is below the sensitization range.
When stainless steel sheet and bar are cold reduced greater than about 30% and subsequently heated to 290 - 425C (550 - 800F), there is a significant redistribution of peak stresses and an increase in both tensile and yield strength. Stress redistribution heat treatments at 290 - 425C (550 - 800F) will reduce movement in later machining operations and are occasionally used to increase strength. Since stress redistribution treatments are made at temperatures below 425C (800F), carbide precipitation and sensitization to intergranular attack (IGA) are not a problem for the higher carbon grades.
Stress relief at 425 to 595C (800 to 1100F) is normally adequate to minimize distortion that would otherwise exceed dimensional tolerances after machining. Only the low carbon "L" grades or the stabilized 321 and 347 grades should be used in weldments to be stress relieved above 425C (800F) as the higher carbon grades are sensitized to IGA when heated above about 425C (800F).
Stress relief at 815 to 870C (1500 to 1600F) is occasionally needed when a fully stress relieved assembly is required. Only the low carbon "L" grades, 321 and 347 should be used in assemblies to be heat treated in this range. Even though the low carbon and stabilized grades are used, it is best to test for susceptibility to IGA per ASTM A262 to be certain there was no sensitization during stress relief treating in this temperature range.
Thermal stabilization treatments at 900C (1650F) minimum for 1 to 10 hours are occasionally employed for assemblies that are to be used in the 400 to 900C (750 to 1650F) temperature range. Thermal stabilization is intended to agglomerate the carbides, thereby preventing further precipitation and intergranular attack (IGA). As with 815 to 870C (1500 to 1600F) stress relief, it is best to test for susceptibility to IGA per ASTM A262.
"Heat Treating, Cleaning and Finishing", Metals Handbook, 10th edition, Vol. 4 in the section entitled "Heat Treatment of Stainless Steels and Heat-Resisting Alloys".
This process is quite different from and not to be confused with the catalytic steam reforming process used industrially to produce various products such as hydrogen, ammonia and methanol from natural gas, naphtha or other petroleum-derived feedstocks. Nor is this process to be confused with various other catalytic reforming processes that use methanol or biomass-derived feedstocks to produce hydrogen for fuel cells or other uses.
Wear is the removal of the material from the surface of a solid body as a result of mechanical action of the counterbody.Wear of Engine bearings is the removal of the bearing material from its surface caused by sliding friction between the bearing and the crankshaft.Wear may combine effects of various physical and chemical processes proceeding during the friction between two counteracting materials: micro-cutting, micro-ploughing, Plastic deformation, cracking, fracture, welding, melting, chemical interaction.The mechanisms of wear:
Abrasive wear
Adhesive wear
Fatigue wear
Corrosive wear
Erosive wear
Abrasive wear
Abrasive wear occurs when a harder material is rubbing against a softer material.
If there are only two rubbing parts involved in the friction process the wear is called two body wear.
In this case the wear of the softer material is caused by the asperities on the harder surface.
If the wear is caused by a hard particle (grit) trapped between the rubbing surfaces it is called three body wear. The particle may be either free or partially embedded into one of the mating materials.
In the micro-level abrasive action results in one of the following wear modes:
Ploughing. The material is shifted to the sides of the wear groove. The material is not removed from the surface.
Cutting. A chip form in front of the cutting asperity/grit. The material is removed (lost) from the surface in the volume equal to the volume of the wear track (groove).
Cracking (brittle fracture). The material cracks in the subsurface regions surrounding the wear groove. The volume of the lost material is higher than the volume of the wear track.
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Adhesive wear
Adhesion wear is a result of micro-junctions caused by welding between the opposing asperities on the rubbing surfaces of the counterbodies. The load applied to the contacting asperities is so high that they deform and adhere to each other forming micro-joints.The motion of the rubbing counterbodies result in rupture of the micro-joints. The welded asperity ruptures in the non-deformed (non-cold worked) regions.Thus some of the material is transferred by its counterbody. This effect is called scuffing or galling.When a considerable areas of the rubbing surfaces are joined during the friction a Seizure resistance (compatibility) seizure of one of the bodies by the counterbody may occur.The factors decreasing adhesive wear:
Lower load.
Harder rubbing materials.
Contaminated rubbing surfaces.
Presence of solid lubricants.
Presence of a lubrication oil.
Anti-wear additives in oil.
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Fatigue wear
Fatigue wear of a material is caused by a cycling loading during friction. Fatigue occurs if the applied load is higher than the fatigue strength of the material.Fatigue cracks start at the material surface and spread to the subsurface regions. The cracks may connect to each other resulting in separation delamination of the material pieces.One of the types of fatigue wear is fretting wear caused by cycling sliding of two surfaces across each other with a small amplitude (oscillating). The friction force produces alternating compression-tension stresses, which result in surface fatigue.Fatigue of overlay of an engine bearing may result in the propagation of the cracks up to the intermediate layer and total removal of the overlay.to top
Corrosive wear
Wear may be accelerated by corrosion (oxidation) of the rubbing surfaces.Increased temperature and removal of the protecting oxide films from the surface during the friction promote the oxidation process. Friction provides continuous removal of the oxide film followed by continuous formation of new oxide film.Hard oxide particles removed from the surface and trapped between the sliding/rolling surfaces additionally increase the wear rate by three body abrasive wear mechanism.to top
Erosive wear
Erosive wear is caused by impingement of particles (solid, liquid or gaseous), which remove fragments of materials from the surface due to momentum effect.Erosive wear of Engine bearings may be caused by cavitation in the lubrication oil. The cavitation voids (bubbles) may form when the oil exits from the convergent gap between the bearing and journal surfaces. The oil pressure rapidly drop providing conditions for voids formation (the pressure is lower than the oil vapor pressure). The bubbles (voids) then collapse producing a shock wave, which removes particles of the bearing material from the bearing.
Grain is a small region of a metal, having a given and continuous crystal lattice orientation. Each grain represents small single crystal.
Grains form as a result of solidification or other phase transformation processes. Grains shape and size change in course of thermal treatment processes (for example recrystallization annealing). The normal grain size varies between 1µm to 1000 µm.
Grain structure of a solid is an arrangement of differently oriented grains, surrounded by grain boundaries. Formation of a boundary between two grains may be imagined as a result of rotation of crystal lattice of one of them about a specific axis.
Depending on the rotation axis direction, two ideal types of a grain boundary are possible:
Tilt boundary – rotation axis is parallel to the boundary plane;
Twist boundary - rotation axis is perpendicular to the boundary plane;
An actual boundary is a “mixture” of these two ideal types.
Grain boundaries are called large-angle boundaries if misorientation of two neighboring grains exceeds 10º-15º.
Grain boundaries are called small-angle boundaries if misorientation of two neighboring grains is 5º or less.
Grains, divided by small-angle boundaries are also called subgrains.
Grain boundaries accumulate crystal lattice defects (vacancies, dislocations) and other imperfections, therefore they effect on the metallurgical processes, occurring in alloys and their properties.
Since the mechanism of metal deformation is a motion of crystal dislocations through the lattice, grain boundaries, enriched with dislocations, play an important role in the deformation process.
Diffusion along grain boundaries is much faster, than throughout the grains.
Segregation of impurities in form of precipitating phases in the boundary regions causes a form of corrosion, associated with chemical attack of grain boundaries. This corrosion is called Intergranular corrosion.
