Materials

The " Seizure " of stainless fasteners between bolt and nut can have different causes:

1. Surface defects in the thread
These are usually caused when rolling threads on already work-hardened surfaces. They are characterized by a cracked, fissured and scaly surface in the thread flanks. These surface defects increase the surface area and thus the attack surface for corrosive influences.

2. Iincorrect tool selection
Under certain circumstances, the tools that are used may not be suitable for processing the material. For example, when cutting a thread, an unsuitable tool may not cut the thread cleanly and completely, resulting in cold welding during assembly. The same applies to the forming of threads. Here, too, the aforementioned situation can occur due to incorrect or defective tools.

3. Hardness difference
One of the most common causes of seizure is a difference in hardness between the screw and nut threads. This difference is essential to prevent the fasteners from permanently bonding ("seizing") through physical-chemical reactions. A hardness difference of at least 50 HV should be aimed for to minimize the risk. As a rule, this is ensured with cold forged fasteners of the same strength class, since the screw thread undergoes significant work hardening in the thread flanks as a result of the forging process. To reduce the risk when using different materials, the delivery states (mechanical properties) of the starting materials should be compared before production.
Additional safety can be achieved by using appropriate sliding pastes, which are applied to the screw thread before assembly. This measure is often used for so-called hot bolted joints to prevent welding of the fasteners at high operating temperatures.

Sources:

Wilke, F. (2002) ThyssenKrupp Steel Kurzbericht 2806 - Wie vermeide ich das Festfressen rostfreier Verbidnungselemente?
Online: http://www.edelstahl-rostfrei.de/downloads/ISER/Festfressen_vermeiden.pdf (05.09.2011)

General

Duplex steels are materials that have a two-phase structure (ferrite and austenite). Duplex steels are characterized by their combination of properties, which are a mixture of the properties of chromium stainless steels (ferritic or martensitic) and chromium-nickel stainless steels (austenitic).

They have higher strengths than chromium-nickel stainless steels, while exhibiting higher ductility than chromium stainless steels. Their behavior under alternating stresses still exhibits fatigue strength up to an austenite content of about 40%, in contrast to pure austenites. Duplex steels are among the rust- and acid-resistant steels.

PREN-Index

The PREN-Index is a measure of the corrosion resistance of a stainless steel. ASTM G48 specifies the test methods for this.

For corrosion resistant steels, the chemical elements that are critical to corrosion behavior are summarized by the PREN, which establishes a relationship between pitting resistance and chemical composition.

PREN = %Cr + 3.3 x %Mo (ferritic steels)

PREN = %Cr + 3.3 x %Mo + 16 x %N (austenitic steels)

PREN = %Cr + 3.3 x %Mo + 30 x %N (duplex steels)

Ferritic-austenitic duplex steels with PREN > 40 are also known as superduplex steels and are characterized by particularly high corrosion resistance. Steels with PREN values above 32 are considered resistant to salt water.

Sources:

DIN EN ISO 15156-3 - Petroleum and natural gas industries-Materials for use in H2S-containing environments in oil and gas production - Part 3: High alloy steels (CRAs) and other alloys
NACE MR0175/ISO 15156 - Petroleum and natural gas industries-Materials for use in H2S-containing environments in oil and gas production
ASTM G48 - Standard Test Methods for Pitting and Crevice Corrosion Resistance of Stainless Steels and Related Alloys by Use of Ferric Chloride Solution

The strength class for bolts consists of two numbers separated by a point. The number on the left corresponds to 1/100 of the nominal tensile strength Rm in MPa. The number to the right of the dot gives 10 times the ratio between yield strength Re and tensile strength Rm (yield strength ratio). This gives the following values: 0.6 / 0.8 / 0.9. These ratios are multiplied by 10 to give the value after the dot.

Example:
A bolt with tensile strength Rm = 1,000 MPa and a yield strength ratio of 0.9 thus has strength class 10.9.

Formula for determining the strength class

Tensile strength Rm: 1st number X 100
Yield strength Re: (1st number X 100) X (2nd number / 10)

Example:
Screw 10.9

Tensile strength Rm: 10 x 100 = 1,000 MPa
Yield strength Re: (10 x 100) x (9 / 10) = 1,000 x 0.9 = 900 MPa

Overview of old and new strength classes

Sources:

DIN 267-3 - Mechanical fasteners; Technical delivery conditions; Strength classes for carbon or alloy steel bolts; Conversion of strength classes.
DIN EN ISO 898-1 - Mechanical properties of carbon and alloy steel fasteners - Part 1: Screws with specified strength classes - Standard and fine pitch threads

Steels with a carbon content of more than 0.22% are only considered to have limited weldability; additional measures such as preheating are required. However, the carbon content of the steel alone does not provide any information on weldability, as this is also influenced by many other alloying elements. The so-called carbon equivalent (CEV) is therefore taken into account for assessment.

In materials science, the carbon equivalent is a measure for assessing the weldability of unalloyed and low-alloy steels. The carbon content and a variety of other alloying elements in the steel influence its behavior. For the purpose of assessing weldability, the carbon equivalent therefore combines the carbon content and the weighted proportion of elements that influence the weldability of the steel in a similar way to that which would be expected from the carbon into a numerical value. A carbon equivalent value of less than 0.45% implies good weldability. Higher values - depending on the processing thickness - require preheating of the material. Above a value greater than 0.65, the workpiece is only suitable for welding with increased effort, since martensite formation can lead to cold or hardening cracks.

A common procedure for calculating the carbon equivalent value (CEV) is as follows:

The alloying elements must be entered as a percentage. The carbon equivalent is often given in the material certificate, so that the user does not have to make this calculation himself. However, under certain circumstances, the usability of the CEV given there for the specific application must be questioned.

Source:

Wittel, H. / Muhs, D. / Jannasch, D. / Voßiek, J. (2009) Roloff/Matek Maschinenelemente - Normung, Berechung, Gestaltung; 19th edition; Vieweg+Teubner Verlag; Wiesbaden

Influence of the elements on the properties of the steel

In the case of alloying elements, a fundamental distinction must be made as to whether they are carbide-, austenite- or ferrite-forming elements, or for what purpose they are added to the steel. Each individual element imparts certain specific properties to the steel depending on its proportion. In the presence of several elements, the effect can be increased. However, there are alloy variants in which the individual elements do not exert their influence in the same direction with regard to a certain behavior, but can counteract each other. The presence of the alloying elements in the steel only provides the prerequisite for the desired properties, which are only achieved through proper processing and heat treatment.

Aluminum (Al): melting point 658°C

It is the strongest, very frequently used deoxidizing and also denitration additive; as a result, it also has a strongly favorable effect on the resistance to aging. In small additions it supports the formation of fine grains. Since aluminum forms high hardness nitrides with nitrogen, it is usually an alloying element in nitriding steels. It increases scale resistance and is therefore often added to ferritic heat-resistant steels. In unalloyed carbon steels, "alitizing" (introducing aluminum into the surface) can be used to promote scaling resistance. Aluminum narrows the gamma range very much. Because of the strong increase in coercive force, aluminum is an alloying element in iron-nickel-cobalt-aluminum permanent magnet alloys.

Arsenic (As): Melting point 817°C under pressure

Also cuts off the gamma area and is a steel damaging element as it shows a strong tendency to segregate, similar to phosphorus. However, elimination of segregation by diffusion annealing is even more difficult than with phosphorus. Furthermore, it increases tempering brittleness, greatly reduces toughness and impairs weldability.

Boron (B): Melting point 2300°C

Since boron has a high effective cross-section for neutron absorption, it is used to alloy steels for regulators and shields of nuclear energy plants. Austenitic 18/8 CrNi steels can be brought to higher yield strength and strength with boron via precipitation hardening, but corrosion resistance is reduced. Boron induced precipitation improves the strength properties of high temperature steels in the elevated temperature range. In structural steels, this element improves through-hardening and thus causes an increase in core strength in case-hardening steels. A reduction in weldability in boron-alloyed steels must be expected.

Beryllium (Be): melting point 1280°C

Copper-beryllium alloys are used to make spiral springs for watches, which are hardly magnetizable and can withstand a much higher number of load cycles than steel springs. Nickel-beryllium alloys are very hard and corrosion resistant; used in surgical instruments. Very strong constriction of the gamma region. Precipitation hardening can be achieved with beryllium, but toughness decreases; strongly deoxidizing, great affinity for sulfur.

Carbon (C): melting point 3540°C

Carbon is the most important and influencing alloying element in steel. In addition to carbon, any unalloyed steel contains silicon, manganese, phosphorus and sulfur, which are added unintentionally during production. The addition of other alloying elements to achieve special effects, as well as the deliberate increase in manganese and silicon content, results in alloyed steel. With increasing carbon content, the strength and hardenability of the steel increase, whereas its elongation, forgeability, weldability and machinability (by cutting tools) are reduced. Corrosion resistance to water, acids and hot gases is practically unaffected by carbon.

Calcium (Ca): melting point 850°C

Used together with silicon in the form of silico-calcium for deoxidation. Calcium increases the scaling resistance of heating conductor materials.

Cerium (Ce): melting point 775°C

Has a purifying effect, as it strongly deoxidizes and promotes desulfurization; it is usually used together with lanthanum, neodymium, praseodymium and other rare precious metals as a "mixed metal". In high-alloy steels, it promotes hot workability in some cases and improves scaling resistance in heat-resistant steels. Iron-cerium alloys with about 70% cerium are pyrophoric (flint). Addition in spheroidal graphite cast iron.

Cobalt (Co): melting point 1492°C

Cobalt does not form carbides; it inhibits grain growth at elevated temperatures and greatly improves temper brittleness and high-temperature strength; therefore, often alloying element in high-speed steels, hot-work steels, high-temperature and high-temperature materials. Favors graphite formation. In large proportions it increases remanence, coercivity and thermal conductivity; therefore alloying base for high-quality permanent magnet steels and alloys. Under neutron irradiation, the highly radioactive isotope 60Co is formed, which is why cobalt is undesirable in steels for nuclear reactors.

Chromium (Cr): melting point 1920°C

Chromium makes steel oil- or air-hardenable. By reducing the critical cooling rate required for martensite formation, it increases hardenability and thus improves temperability. Notched impact strength, however, is reduced, but it lowers elongation only very slightly. Weldability decreases with increasing chromium content in pure chromium steels. The tensile strength of the steel increases around 80-100 N/mm2 per 1% chromium. Chromium is a carbide former. Its carbides increase cutting ability and wear resistance. High-temperature strength and resistance to hydrogen under pressure are favored by chromium. While increasing chromium contents increase scale resistance, a minimum content of about 13% chromium is required for corrosion resistance of steels, which must be dissolved in the matrix. The element cuts off the gamma region, thus extending the ferrite range; but stabilizes the austenite in austenitic chromium-manganese or chromium-nickel steels. Thermal conductivity and electrical conductivity are reduced. Thermal expansion is lowered (alloys for glass melting). With a simultaneously higher carbon content, chromium contents of up to 3% increase remanence and coercivity.

Copper (Cu): melting point 1084°C

Copper is only added to a few steel grades because it accumulates under the scale layer and, by penetrating the grain boundary, causes great surface sensitivity during hot forming processes, which is why it is sometimes regarded as a steel pest. Yield strength and yield strength ratio are increased. Contents above 0.30% can cause hardening. Hardenability is improved. Weldability is not affected by copper. In unalloyed and low-alloy steels, a significant improvement in weather resistance is achieved by copper. In acid-resistant high-alloy steels, a copper content above 1% provides improved resistance to hydrochloric acid and sulfuric acid.

Hydrogen (H): Melting point -262°C

This element is a steel pest because it causes embrittlement due to a drop in elongation and necking without an increase in yield strength and tensile strength. Hydrogen is the cause of the dreaded flake formation and favors the formation of shadow streaks. Atomic hydrogen generated during pickling penetrates the steel forming bubbles. Moist hydrogen decarburizes at higher temperatures.

Magnesium (Mg): melting point 657°C

This element is added as a deoxidizing and desulfurizing agent. In cast iron, magnesium produces nodular graphite.

Manganese (Mn): melting point 1221°C

Manganese deoxidizes. It bonds sulfur as manganese sulfides and thus reduces the unfavorable influence of the iron sulfide. This is of particular importance in free-cutting steel; the risk of red fracture is reduced. Manganese reduces the critical cooling rate very considerably and thus increases hardenability. Yield strength and strength are increased by manganese addition. Furthermore, manganese has a favorable effect on forgeability and weldability and greatly increases the hardening depth. Contents above 4% lead to the formation of a brittle martensitic microstructure even during slow cooling, so that the alloying range is hardly used. Steels with manganese contents above 12% are austenitic with a simultaneously high carbon content, because manganese expands the gamma range considerably. Such steels obtain a very high work-hardening under impact stress on the surface, while the core remains tough; they are therefore highly wear-resistant under impact. Steels with manganese contents of 18% upwards remain non-magnetizable even after relatively severe cold working and are used as special steels and also as cold-hardening steels for low-temperature stressing. Manganese increases the coefficient of thermal expansion, while thermal conductivity and electrical conductivity decrease.

Molybdenum (Mo): Melting point 2622°C

Molybdenum usually alloyed with other elements. By reducing the critical cooling rate, hardenability is improved. Molybdenum largely reduces tempering brittleness, for example in chromium, nickel and manganese steels, promotes fine grain formation and has a favorable effect on weldability. Increase in yield strength and strength. With higher molybdenum content, cuttability is impeded. Strong carbide former; it improves cutting properties in fast-working steels. It belongs to any element that increases corrosion resistance and is therefore widely used in high-alloy chromium steels and in austenitic chromium-nickel steels; high molybdenum contents reduce the susceptibility to pitting. Very strong narrowing of the gamma range; increase in high-temperature strength, scaling resistance is reduced.

Nitrogen (N): Melting point -210°C

This element can appear both as a steel pest and as an alloy component. Harmful because of reduction of toughness by precipitation processes, induction of ageing sensitivity and blue brittleness (deformation in areas of blue heat 300-350°C), and the possibility of inducing intergranular stress corrosion cracking in unalloyed and low-alloy steels. As an alloying element, nitrogen extends the gamma region and stabilizes the austenitic microstructure; in austenitic steels, it increases strength and, above all, yield strength and mechanical properties in heat. Nitrogen allows high surface quality to be achieved through nitride formation during nitriding.

Niobium-tantalum (Nb-Ta): Melting points Nb 1960°C Ta 3030°C

These elements occur almost only together and are very difficult to separate from each other, so they are usually used together. Very strong carbide formers, therefore alloyed especially as stabilizers chemically resistant steels. Both elements are ferrite formers and thus reduce the gamma range. As a result of the increase in high-temperature strength and creep rupture strength due to niobium, it is often added to high-temperature austenitic boiler steels. Tantalum has a high absorption cross-section for neutrons; only tantalum-poor niobium can be considered for nuclear reactor steels.

Nickel (Ni): melting point 1453°C

Increases yield strength and impact toughness in structural steels. Nickel is also used in case-hardening and quenched and tempered steels to increase toughness. The element extends the gamma region and therefore effects the austenite structure in corrosion and scale resistant chromium-nickel steels. High nickel contents lead to steels with small temperature expansion (e.g. Invar).

Antimony (Sb): Melting point 630°C

This element is a steel pest, it lowers the toughness of the steel; it cuts off the gamma region. In general, little information is available about this alloying element.

Lead (Pb): Melting point 327.4°C

Lead is actually not a "real" alloying element, since it hardly influences the given properties at certain alloying contents. This alloying element is added in contents of approx. 0.2%-0.5% to significantly improve machinability. Shorter chips and clean cutting surfaces are produced. Another application of lead is in bearings, where its excellent sliding properties are exploited.

Phosphor (P): Schmelzpunkt 44°C

This element is mostly a steel pest and strongly alloyed. However, phosphorus is often used in small quantities in free-cutting steels. Phosphorus is usually considered as a steel pest, as it gives strong primary segregations during solidification of the melt and the possibility of secondary segregations in the solid state due to the strong constriction of the gamma region. As a result of the relatively low diffusion rate, both in the alpha and in the gamma solid solution, given segregations can only be compensated with difficulty. Since it is hardly possible to achieve a homogeneous distribution of the phosphorus, one tries to keep the P content very low and accordingly to aim at an upper limit of 0.03%-0.05% for high-quality steels. The extent of segregation cannot be determined with certainty Phosphorus increases sensitivity to tempering embrittlement even at the lowest levels. P embrittlement increases with increase in C content, with increasing hardness temperature, with grain size and with reduction in degree of forging. The embrittlement appears as cold brittleness and sensitivity to impact stress. In low-alloy structural steels with C contents of about 0.1%, phosphorus increases strength and corrosion resistance to atmospheric effects; Cu assists in improving corrosion resistance (stainless steels). Phosphorus additions can cause yield strength increases and precipitation effects in austenitic chromium-nickel steels.

Oxygen (O): Melting point -218.7°C

This element is a steel pest. Oxygen deteriorates the technological properties notch toughness and aging. In addition, the element produces red fracture and promotes wood fiber breakage.

Sulfur (S): melting point 118°C

This element is the most alloying. Iron sulfide, reinforced by oxygen, leads to red fracture. In addition, the toughness is deteriorated. Sulfur in free-cutting steels up to 0.3% increases machinability.

Silicon (Si): melting point 1414°C

This element has a deoxidizing effect and narrows the gamma range. Silicon increases strength and wear resistance. This alloying addition also greatly increases the elastic limit, so it is added to spring steels. At high levels of silicon, the addition increases scaling resistance and acid resistance, but decreases electrical conductivity and coercivity; therefore, silicon is used in electrical sheets.

Titanium (Ti): melting point 1727°C

This element is a strong deoxidizer and carbide former. Therefore, titanium is often used as a stabilizer in corrosion-resistant steels.

Vanadium (V): melting point 1726°C

This element is a strong carbide former. Vanadium binds nitrogen and produces a fine-grained cast structure. It increases wear resistance due to hard carbides, as well as hot strength and temper resistance. Vanadium is therefore added to high-speed steel, hot-work tool steel and high-temperature steel. It also increases the elastic limit in spring steels.

Tungsten (W): melting point 3380°C

Tungsten increases tensile strength, yield strength and toughness. It is also a strong carbide former (hard carbides). Tungsten increases hot strength and wear resistance, so it is used as an additive in high speed steel and hot work tool steel.

Tin (Sn): Melting point 232°C

This element is a steel pest and alloy strongly.

Zircon (Zr): melting point 1860°C

In special cases, this element is used as an additive element for deoxidation, dendrification and desulfurization. It is a strong carbide former. Zr additions to fully killed free-cutting steels containing sulfur exert a favorable influence on sulfide formation and thus avoidance of red fracture. It increases the service life of heatsealing band materials and causes a narrowing of the gamma range.

Source:
This information was kindly provided by the publisher Stahlschlüssel Wegst GmbH. Further information at http://www.stahlschluessel.de.