what is heat treatment scaling

Heat treat scale is an undesired, dark, usually iron-rich, flaky material on the surface of the material being heat-treated. During subsequent machining this scale can reduce the life of, and/or damage machining tooling, and may cause serious surface defects, on a finished part.

Full Answer

What is scaling in water heating and cooling systems?

Scaling and fouling will increase the system’s resistance to heat transfer, which is undesirable for water heating and cooling equipment. Even thin layers of scale will create effective insulation since the percentage of energy required to heat or cool the water increases as scale buildup does.

What is scaling in heat exchangers?

Scaling occurs when a mineral film coats the entire surface of a heat exchanger. The most common forms of scale are usually from calcium-based salts such as calcium sulfate or calcium carbonate. What''s so special (or problematic) about these calcium-based salts?

What is heat treatment?

Heat treatment involves heating of metal in the solid-state and then subsequently cooled at varied cooling rates. It is very important manufacturing process that can not only help the manufacturing process but can also improve the product, its performance, and its characteristics in many ways.

What are the specifications of a heat treatment?

Good heat treatment specifications include the material as expressed in the standards—AISI 1040 for a carbon steel, for example, or SAE 4140 for an engineering alloy. It’s not enough to tell a heat treater you’d like a harder part because there are many ways to do that. Does it need to be through hardened? Case hardened?

How do you prevent heat treatment scaling?

To prevent scaling and decarburization, care is taken to apply a uniform coating layer on the component. The coating also reduces decarburization on billets and ingots during hot-forging and hot-rolling operations. Heat transfer from the heating medium to the metal is unaffected by the coating.

What are the 3 stages of heat treatment process?

Three stages of heat treatment The stages of the heat treatment process include heating, soaking, and cooling.

What is the difference between the heat treatment and hardening?

Main Difference – Annealing vs Hardening vs Tempering Annealing is a heat treatment process used to soften materials or to obtain other desired properties such as machinability, electrical properties, dimensional stability, etc. Hardening or quenching is the process of increasing the hardness of a metal.

What is tempering heat treatment process?

tempering, in metallurgy, process of improving the characteristics of a metal, especially steel, by heating it to a high temperature, though below the melting point, then cooling it, usually in air. The process has the effect of toughening by lessening brittleness and reducing internal stresses.

What is the main purpose of heat treatment?

Heat treating can soften metal, to improve formability. It can make parts harder, to improve strength. It can put a hard surface on relatively soft components, to increase abrasion resistance. It can create a corrosion-resistant skin, to protect parts that would otherwise corrode.

Why is heat treatment necessary?

Heat treating can improve wear resistance by hardening the material. Metals (including steel, titanium, inconel, and some copper alloys) can be hardened either on the surface (case hardening) or all the way through (through hardening), to make the material stronger, tougher, more durable and more resistant to wear.

What happens during heat treatment?

Heat treatment is the process of heating metal without letting it reach its molten, or melting, stage, and then cooling the metal in a controlled way to select desired mechanical properties. Heat treatment is used to either make metal stronger or more malleable, more resistant to abrasion or more ductile.

What are the five basic heat treatment process?

Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering, carburizing, normalizing and quenching.

How many types of heat treatment are there?

What are the 4 Types of Heat Treating Processes? Common types of heat treating methods include annealing, hardening, quenching, and stress relieving, each of which has its own unique process to produce different results.

What is the difference between heat treating and tempering?

Both heat treatments are used for treating steel, although annealing creates a softer steel that is easier to work while tempering produces a less brittle version that is widely used in building and industrial applications.

What is annealing vs tempering?

Annealing involves heating steel to a specified temperature and then cooling at a very slow and controlled rate, whereas tempering involves heating the metal to a precise temperature below the critical point, and is often done in air, vacuum or inert atmospheres.

What is normalizing and tempering?

Tempering treatment means that the steel after quenching hardening or normalization treatment is cooled down at a certain rate after being immersed for a period of time below the critical temperature.

Why is heat treatment called an arrest?

This temperature is referred to as an "arrest" because at the A temperature the metal experiences a period of hysteresis.

What is the process of heating something to alter it?

Process of heating something to alter it. Heat treating furnace at 1,800 °F (980 °C) Heat treating (or heat treatment) is a group of industrial, thermal and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the ...

How much carbon is in hypoeutectoid steel?

A hypoeutectoid steel contains less than 0.77% carbon. Upon cooling a hypoeutectoid steel from the austenite transformation temperature, small islands of proeutectoid-ferrite will form. These will continue to grow and the carbon will recede until the eutectoid concentration in the rest of the steel is reached.

How does steel change carbon?

When steel is heated in an oxidizing environment, the oxygen combines with the iron to form an iron-oxide layer, which protects the steel from decarburization. When the steel turns to austenite, however, the oxygen combines with iron to form a slag, which provides no protection from decarburization. The formation of slag and scale actually increases decarburization, because the iron oxide keeps oxygen in contact with the decarburization zone even after the steel is moved into an oxygen-free environment, such as the coals of a forge. Thus, the carbon atoms begin combining with the surrounding scale and slag to form both carbon monoxide and carbon dioxide, which is released into the air.

Does cooling a metal cause precipitation?

Cooling a metal will usually suppress the precipitation to a much lower temperature. Austenite, for example, usually only exists above the upper critical temperature. However, if the austenite is cooled quickly enough, the transformation may be suppressed for hundreds of degrees below the lower critical temperature.

Why is heat treatment important?

It is very important manufacturing process that can not only help the manufacturing process but can also improve the product, its performance, and its characteristics in many ways. By Heat Treatment process, Example: The plain carbon steel. The following changes may be achieved: The hardness of Steel may be increased or decreased.

What is normalizing steel?

Normalizing is a heat treatment process similar to annealing in which the Steel is heated to about 50 degree Celsius above the upper critical temperature followed by air cooling. This results in a softer state which will be lesser soft than that produced by annealing.

What are the changes in steel?

The following changes may be achieved: The hardness of Steel may be increased or decreased. Internal stresses that are set up due to cold or hot working may be relieved. The machinability of Steel may be enhanced. The mechanical properties like tensile strength the Talati shock resistance toughness etc may be improved.

What is the purpose of hardening steel?

Hardening is carried to accomplish the following: To reduce the grain size. Obtain maximum hardness.

What is recrystallization in steel?

This causes complete recrystallization in steel to form New grain structure. This will release the internal stresses previously the strip in the steel and improve the machinability.

What is normalizing carried for accomplishing?

Normalizing carried for accomplishing one or more of the following: To refine the grain size. Reduce or remove internal stresses. Improve the machinability of low carbon steel. Increase the strength of medium carbon steel. And also To improve the mechanical properties of the medium Carbon Steel.

What temperature does annealing take place?

Annealing consists of heating of steel parts to a temperature at or near the critical temperature 900 degree Celsius hold it at that temperature for a suitable time and when allowed to cool slowly in the Furnace itself. The heating done during annealing affects the metal in two stages of recovery and recrystallization.

Clearly Identified Materials

The chemical makeup of a part is one of the most critical determinants of how it is heat treated. It’s not enough to state on the spec that a piece is steel alloy.

Specific Process Required

It’s not enough to tell a heat treater you’d like a harder part because there are many ways to do that. Does it need to be through hardened? Case hardened? Does it require stress relief via annealing? Specs that dictate which process is to be used help heat treaters shape the rest of the heat treatment steps that follow.

Hardness Tolerance

For through hardened parts, a prescribed hardness should be included on the spec and expressed as a range.

Case Depth Tolerance

For case hardened materials (i.e., those that are carburized or carbonitrided), specs should indicate whether the desired hardness is expressed as effective case depth or total case depth. Total case depth refers to the distance carbon has diffused into the part. This is usually specified for parts that have thinner case depths after treatment.

Avoid Too Much Information

Sometimes, though, too much specificity can lead to trouble.

Correct Hardness Scales

The scale on which a part’s hardness is determined depends on the heat treatment applied to the part. In the U.S., we typically use the following four hardness scales: Rockwell Hardness, Brinell Hardness, Microhardness and Leeb Hardness.

Inspection Points

Heat treatments are carefully designed to achieve specific results on specific areas of parts, so owners need to clearly identify those areas on which hardness tests are to be conducted. For example, the critical part of the theoretical gear mentioned above is its teeth; case hardening is designed to strengthen that part of the gear while leaving other areas relatively soft and ductile.

Why do metalurgists need accurate specifications?

Metallurgists need accurate specifications in order to correctly perform the necessary heat treatment of parts. This helpful guide, written by William Rassieur, Sales Leader at Paulo Heat Treating, is a useful tool to identify what details ought to be communicated to the heat treating expert.

What is the total case depth?

Total case depth refers to the distance carbon has diffused into the part. This is usually specified for parts that have thinner case depths after treatment. Effective case depth applies to parts with generally thicker cases. This is measured as the distance from the surface through the case to a specific hardness level. Usually, that hardness is effective based from 50 or 52 HRC. This should always be stated on specs.

Is it enough to state on a spec that a piece is steel?

It’s not enough to state on the spec that a piece is steel alloy. Consult materials standards and use the correct material designation on the spec. For example, if you want to treat a carbon steel or an engineering alloy, using those terms (or known trade names for a specific material) isn’t adequate.

What temperature does calcium plating out occur?

For most calcium based salts, this "plating out" effect begins to occur around 140 °F and get even worse at higher temperatures. What makes scaling so difficult to deal with is that there nothing that can be done from a design stand point. No amount of shear rate/velocity to stop this from happening!

Why is fouling important in heat exchangers?

Both fouling and scaling essentially provide an additional (and often times significant) resistance to heat transfer in your heat exchanger. In plate heat exchangers, the degradation of performance is usually magnified because plate exchanger depend on a thin plate wall to maintain their high efficiencies. Often times, engineers will choose plate ...

What happens when you heat a water based stream containing salts?

So what happens when you heat a water-based stream containing these salts is that the salts begin to plate out onto the heat transfer surfaces. Once this happens, the heat transfer performance of your heat exchanger will decrease quickly.

What happens if a plate heat exchanger is poorly designed?

However, if a plate heat exchanger is poorly designed or is run at flow rates more than 20% below their intended design, then this shear rate can be reduced to a point where fouling can certainly occur.

What is scale in heat exchangers?

Scale is an unwanted material that accumulates onto the internal surfaces of a heat exchanger — this deposition is otherwise known as fouling. If untreated, fouling will harden and prevent the system from operating at its intended, energy-efficient state. Scaling and fouling will increase the system’s resistance to heat transfer, ...

How to mitigate scale buildup?

The most effective way to mitigate scale buildup is to prevent its formation in the first place. Scale can be controlled in a heat exchanger by: Pretreating boiler make-up water: Demineralizers, water softeners and reverse osmosis can target and remove minerals that are likely to form scale.

Why is it important to clean heat exchangers?

Periodic heat exchanger cleanings are essential because scale formation can quickly result in overheating, tube failures, increased energy consumption and a rise in operational costs. Scale may even corrode equipment if left without proper care, requiring intensive repairs or total replacement to correct.

What is CA 100 used for?

CA-100 should be used for heat exchangers with stainless steel components. The citric acid is less aggressive and will not pit stainless steel. Ensure the chosen product will completely dissolve the scale by performing a preliminary test on a sample of the deposit.

What is the most common form of scale?

Scale comes in a variety of forms, but the most common is calcium carbonate or CaCO3. Calcium scale precipitates when the threshold solubility of calcium and carbonate is exceeded. Scale occurs when the dissolved minerals found in a water source fall out of suspension and form a continuous deposit layer on the surface of a heat exchanger.

Does scale increase heat transfer?

Scaling and fouling will increase the system’s resistance to heat transfer, which is undesirable for water heating and cooling equipment. Even thin layers of scale will create effective insulation since the percentage of energy required to heat or cool the water increases as scale buildup does.

Overview

Specification of heat treatment

Physical processes

Effects of composition

Effects of time and temperature

Techniques

Furnace types

See also