Oil and gas, petrochemical, chemical and power generation plants contain boilers, pressure piping vessels, valves and process equipment requiring high-strength bolting to maintain seal closure on flanged joints, fittings and closures. Heat-treated bolts are needed to withstand the tensile stresses within the bolts.
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How to heat treat bolts and fasteners for demanding applications?
Heat Treatment of Bolts and Fasteners for Demanding Applications 1 Heat Treatment Processes. About 90 percent of fasteners are steel based and the required strength level is usually developed in steel fasteners using quenching and tempering processes. 2 Proper Use and Maintenance. ... 3 Conclusion. ...
What are the common problems with heat treated bolts?
Equipment failure is a strong possibility if heat-treated or high-strength bolts are replaced with softer bolts during an overhaul, retrofit or upgrade operation. In MRO applications, the markings on the bolts should be noted when replacing worn, damaged or rusted bolts.
How much torque do I need to tighten a flange bolt?
The assembly torque that is recommended for high tensile 20- mm bolts to give 75% yield stress is about 550 Nm. A man tightening a 20-mm diameter flange bolt with a 300-mm, or even a 450-mm, long spanner would probably not generate enough force to tension the bolts.
What is heat treatment for stainless steel bolts?
Heat treatments of stainless bolts are employed to dissolve coarse chromium carbide particles, which may reduce corrosion resistance and toughness. The carbide solution treatment of stainless steel fasteners can improve cryogenic or low temperature toughness and is part of the ASTM A320 specification.
How are bolts heat treated?
Quench & Temper Hardening Process In the quench and temper hardening process, steel fasteners are first heated up to a temperature where their structure is converted into austenite and then quenched or rapidly cooled in water, oil or air to crystal structures that transform to martensite.
What can go wrong during heat treatment?
MOST OF THE PROBLEMS in heat- treated parts are attributed to faulty heat- treatment practices (such as overheating and burning, and nonuniform heating and quench- ing), deficiency in the grade of steels used, part defect, improper grinding, and/or poor part design.
How can we prevent distortion during heat treatment?
Press quenching is a specialized quenching technique that can be used to minimize the distortion of complex geometrical components during heat treatment.
Why must a high strength steel screw go through heat treatment after electroplating process?
The plating process is thought to be a principal cause of hydrogen embrittlement: fasteners can fail unpredictably at applied stress levels well below the fracture stress. To avoid this a hydrogen relief heat treatment after electroplating is commonly applied.
What happens if you don't heat treat steel?
Watch For Brittleness If you don't temper your metal properly, you'll expose your metal products to potential brittleness. Brittleness occurs when metal is tempered for too short of a time.
What are the five basic heat treatment process?
Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering, carburizing, normalizing and quenching.
Why do blades warp when quenched?
When these heated parts are quenched, their internal crystal structure changes again, and that volume change is not necessarily sufficient to offset the change upon heating. This change of volume can cause dimensional distortion.
How does heat treating affect steel?
Heat Treating of steel and other metals can lead to: Improved wear resistance. Increased resistance to deformation and warpage and. Increased strength or toughness.
What are the three stages of heat treatment?
Stages of Heat TreatmentThe Heating Stage.The Soaking Stage.The Cooling Stage.
What causes embrittlement?
Embrittlement is a series complex mechanism that is not completely understood. The mechanisms can be driven by temperature, stresses, grain boundaries, or material composition. However, by studying the embrittlement process, preventative measures can be put in place to mitigate the effects.
What can cause hydrogen embrittlement?
Hydrogen Embrittlement occurs when metals become brittle as a result of the introduction and diffusion of hydrogen into the material. The degree of embrittlement is influenced both by the amount of hydrogen absorbed and the microstructure of the material.
Why is hydrogen embrittlement needed?
Hydrogen embrittlement is a metal's loss of ductility and reduction of load bearing capability due to the absorption of hydrogen atoms or molecules by the metal. The result of hydrogen embrittlement is that components crack and fracture at stresses less than the yield strength of the metal.
Why use heat treated fasteners?
Heat-treated fasteners are also essential in engine, vehicle and hydraulic applications where bolts and fasteners are under high stresses. For instance, high-strength head, block and other bolting on large diesel marine and truck engines are highly torqued or pre-stressed to keep the gaskets sealed. High pressure and four bolt hydraulic flange ...
What is a heat treated bolt?
Heat-treated bolts are stronger, but not indestructible. Misuse of fasteners could lead to their breakage and even failure of the entire assembly, equipment or structure. Over-tightening hardened bolts above their torsional strength can result in failure. Figure 6. Heat-treated carbon, boron and alloy steel bolt grade identification marks. Under-tightened fasteners can lead to premature failure due to fretting, fretting fatigue or changes in loading and internal stresses on the fasteners. As with any threaded fastening system, the proper torque level must be applied to take full advantage of the bolt’s strength levels. If a soft, non-heat treated nut is used with a hardened bolt, then premature failure is likely to occur in the nut. In bolting applications, ASTM A563 or ASTM A194 grade heavy-duty nuts should be used with strength levels comparable to or compatible with the high-strength bolts selected (Figure 5). Heat-treated bolts are not designed for use in threaded holes because steel plates or structural members are usually soft carbon steel or unhardened alloy steel. In threaded stud applications, the material with the threaded hole should have strength levels compatible with the heat-treated fastener. Washers should meet ASTM F436 specifications for hardened steel washers as well.
What to look for when replacing a rusted bolt?
In MRO applications, the markings on the bolts should be noted when replacing worn, damaged or rusted bolts. If the bolt head is rusted or damaged to the point where the markings are unreadable, the equipment manual or OEM should be consulted for the proper fastener specifications. If the bolts were at the point of failure in the repair project, then consulting the equipment manual, construction specifications or OEM for the correct bolt grades would be a good idea. If no markings are visible, manuals are lost and the manufacturer is unknown or closed, then as a last resort, hardness testing of the bolt might indicate if the fasteners being replaced were heat treated.
What is the purpose of carbide treatment on stainless bolts?
Heat treatments of stainless bolts are employed to dissolve coarse chromium carbide particles, which may reduce corrosion resistance and toughness. The carbide solution treatment of stainless steel fasteners can improve cryogenic or low temperature toughness and is part of the ASTM A320 specification.
What is case hardening?
Case hardening is a process for hardening a surface or “case” layer of low carbon and alloy steels. The steel is first carburized, or carbonitrided, to increase the carbon content in the outer layer or case. The carburized steel pin or fastener is then conventionally hardened with a quench and temper process.
Why are high strength fasteners needed?
High-strength or heat-treated fasteners are often required to handle the tensile, shear or combination loads on the bolted joints in demanding applications. Many demanding applications require high-strength fasteners in order to handle high tensile and shear stresses generated by the applied loads on the bolted or fastened joint.
What are anchor bolts made of?
Structural anchor bolts are made of carbon or fully heat-treated medium alloy steel.
What is heat treated bolt?
Heat treated bolts are put through one of the following processes to achieve optimal performance in their given applications: Quenching – Quenching can be called the most essential step in heat treatment. It is designed to create a super-saturated solid solution at room temperature.
What is aerospace bolt?
Aerospace bolts are one of the most critical components in any aircraft, rotorcraft or spacecraft. They must meet incredibly demanding requirements in order to perform in such extreme environments.We create aerospace bolts from many superalloys and exotic materials.
What is solution heat treatment?
Solution heat treatment is a complicated process. Overheating and underheating can cause a loss in tensile strength, fracture toughness, and ductility. Precipitation heat treatments – Precipitation heat treatments provide hardening by precipitation constituents from solid solution.
What is the process of annealing aluminum?
Annealing – Annealing is a process used to relieve stresses, to stabilize properties and dimensions, to increase ductility, and to soften strain-hardened and heat treated superalloys — but at a loss of strength.
How does age hardening work?
Age hardening – Age hardening can be achieved in two ways: naturally and artificially. Natural age hardening refers to the hardening that occurs naturally after quenching. The degree of hardening depends on the alloy. Artificial age hardening refers to the process of reheating quenched materials to around 212 degrees Fahrenheit ...
Is aluminum alloy heat treated?
Numerous aluminum alloys exist, and some like aluminum 7075 are nearly as strong as steel. All aluminum alloys are classified as either heat treatable or not heat treatable. Heat treated bolts are put through one of the following processes ...
What are the three categories of heat treatment for fasteners?
Fastener heat treatment can be divided into the three categories of steel, steel surface treatment and aluminum. Heat treatment of steel will be covered here. Steel surface heat treatment and aluminum heat treatment will be covered in Part 2.
How does hydrogen embrittlement occur?
Hydrogen embrittlement occurs due to electrochemical surface treatments such as plating. Hydrogen in atomic form enters the molecular lattice of the steel and causes microcracks to form (Fig. 1), which causes mechanical failure. It is prevented by heating the material at temperatures of 177-221°C (350-430°F) for a period of 8-24 hours. It is important to perform the baking process as soon as possible after plating. Otherwise, the hydrogen will become fully absorbed into the steel lattice, preventing it from being baked out. It is common practice to begin the baking process within three hours of plating, and many manufacturers specify a maximum of one hour. Due to the characteristics of the hydrogen and the steel lattice, higher-strength steels are more susceptible to hydrogen embrittlement.
What happens to martensite during hardening?
It releases the internal stresses induced during the hardening process. Tempering transforms the structure into tempered martensite, which is both hard and ductile but not brittle. Some hardness is lost and the toughness is improved, achieving a significant gain in properties.
What temperature is normalizing steel?
Normalizing is also typically performed on raw material (e.g., wire, rod, bar) and is done at approximately 19-38°C (50-100°F) above the upper critical temperature (Ac 3 or Ac m) of the steel. Similar to annealing, normalizing consists of heating to the desired temperature, soaking at the temperature for a specified time and then cooling to room temperature.
What is subcritical annealing?
Subcritical annealing of steel (i.e., stress-relief annealing) is performed on the raw material to reduce stress by allowing it to recrystallize. It relieves the stresses generated during cold working, increases ductility and toughness, and produces the desired microstructure.
Is heat treatment different for steel?
It is important to note that the heat treatment of steel and alloy fasteners is different than for aluminum fasteners. The heating temperature, time and quench process vary depending on the fastener material, the desired material properties and the production rate. Fastener heat treatment can be divided into the three categories of steel, ...
What is the most inaccurate method to use to tighten bolts?
Tightening bolts by ‘feel’ is the most inaccurate method to use.
What is the purpose of bolt tension?
Bolts of sufficient tensile strength to take the process and bolting forces . Bolt tension must allow for cycling of loads, shock loads, shear loads and vibration. When pulling up bolts try to apply the torque evenly and continuously. Use washers under the bolt head and nut.
What happens if a bolt spring is too loose?
If the ‘bolt-spring’ is too loose, the pressure stretches the bolt and the flange opens and leaks. To prevent the flanges separating, the bolts are preloaded (stretched). Bolt torque figures are calculated to produce a bolt stretched to at least 65% of its yield strength.
How to make a bolt tighter?
Swap the bolt for one of higher tensile strength and pull it up tighter. Make sure all the bolts are to the same rating and diameter. Lubricate the bolt threads lightly and follow the recommended bolting-up sequence.
What does it mean when a gasket springs?
A ‘gasket-spring’ will creep (squash) with time or temperature. Gasket creep is the gradual loss of the gasket’s ‘springiness’. The spring tension in the gasket slackens off. Gasket creep may require that flanges be re-tensioned periodically.
What affects the amount of torque needed to preload a bolt?
The surface finish of the bolt and nut threads also affects the amount of torque needed to preload (stretch) the bolt.
What is the standard thread size for galvanized thread?
They vary from 2.1 for a dry galvanized thread to 0.7 for a nicely machined and oiled thread.
Why 304 material is not recommended for bolts in flange joint sealing
In flange joint sealing, when carbon steel flange or stainless steel flange is equipped with 304 bolts, leakage often occurs in operation. This lecture will make a qualitative analysis of this.
What are the basic differences between 304, 304L, 316 and 316L materials?
304, 304L, 316 and 316L are commonly used stainless steel materials in flange joints (including flange, sealing element and fastener ).
Why should 304 bolts and 316 bolts not be used for flange joints?
As mentioned in the previous sections, firstly, the internal pressure causes the separation of the two flange sealing surfaces of the flange joint, resulting in the corresponding reduction of the gasket stress; secondly, the creep relaxation of the gasket or the bolt itself causes the bolt stress relaxation under high temperature, which also causes the gasket stress to drop, resulting in the leakage failure of the flange joint..