As mentioned above, the heat must be even throughout the part to be quenched. The effect of heat on steel is to expand it, thus uneven thickness or asymetric shapes may cause a part to heat unevenly. When this happens expansion of the part is also uneven. This is where distortion will take place and remain permanent.

We could get very deep in the weeds about each of these and so for the sake of brevity, we should just be satisfied with a general understanding that a combination of these things happen during the rapid cooling of carbon steels which causes the creation of martensite, prevents slip, and makes the steel hard.

Steel has been an integral part of the formation of our society and continues to play a major role in our industrial development. The two major reasons for this is because of its abundance and versatility.

The best way to think about heat treating is to consider the shifting of the crystalline structure from one phase to another. In the case of Hardening, martensite is the phase that the steel has transformed into.

Metalurgist agree on several common factors in the creation of martensite, but cannot pinpoint one over another as to the primary cause of it. The factors are: Distortion of the crystalline lattice causing internal stress, the dramatic reduction of grain size, the creation of submicroscopic particles of cementite, the shift of carbon atoms into an intralattice structure, and suppression of expansion from lattice distortion.

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The most common and hence most frequently machined stainless steels are the austenitic types, such as grades 304 (1.4301) and 316 (1.4401). These are characterised by their high work hardening rates and poor chip breaking properties during machining. This article covers the important issues that influence the successful machining of these steels.

The shape of the part is an important factor to consider when quenching. A long part quenched vertically (long-wise) will cool first at the dipped end before the whole part is submerged. This will create radical unevenness and will likely result in distortion or breakage. The same part turned 90 degrees will submerge much quicker and cool more evenly. Asymetric parts should have their thickest end quenched first. e.g. knife blades and cutting tools.

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When starting a project, knowing the type of steel is ideal. If the material is new, then the manufacturer can supply you with all the material data that you require for a successful heat treat. If the steel has been reclaimed from the scrap yard, then making an attempt to identify it is strongly recommended.

It is essential to keep the cutting tools sharp when machining stainless steels. Careful grinding and honing of the tool faces to give accurate and sharp face angles is important. This helps optimise:

The lubrication provided by cutting fluids also helps reduce tool wear and wash away the machining swarf. Generally cooling is more important than lubrication with faster the cutting speeds and so high cutting fluid flow rates are normally used when machining stainless steels.

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Some ‘squealing’ as the metal is being cut is not unusual, but can indicate that the tool may be wearing and need replacing.

Correct tool geometry is important for minimising swarf build up on the tool faces. Swarf build up can also result in increased machine power requirements and poor surface finish on the machined surfaces. Tool relief angles must be flat. Concave relief faces can result in tool chipping or breakage due to the reduced support of the cutting edge.

Millingstainless steelspeeds and feeds

For this reason selecting a carbon steel that is appropriate for your application is critical. Using steels that are not hard enough will result in marring of the tool. Using steels that are too hard will crack and break. Both can be dangerous, not to mention a waste of your time.

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Water is a very useful quenching medium because it is abundant (if you don't live in California), easily attainable and inexpensive. It is also desirable because it can be disposed of without any environmental or pollution issues. Plain water offers close to the near maximum rate of cooling available in a liquid and works well to pop scale off steel as it is being quenched.

This is just as important in the heating process as it is for quenching. Lets say there is a part with a thin edge on one side. Care must be taken as to not overheat the thin part and melt it. Likewise, that same part must be positioned in the fire so that it reaches critical temp evenly (this may require rotating and flipping). Finally, these same considerations must be made when quenching lest any disproportion in the thickness of the part lead to uneven cooling, distortion, and cracking.

One drawback is that the rate of cooling is so dramatic, that it can cause distortion and even cracking. Because of this, water is typically reserved for quenching simple, symmetrical parts of lower hardening grades of carbon steel.

When speaking of Hardening, we focus on two terms: martensite and the prevention of slip. Martensite is the hardest state the crystalline structure can have. It is created when the steel is rapidly cooled from its critical temp to below 617 degrees F. Slip is the word used to describe the movement within the crystalline structure along tiny gaps between the grains of the metal. This is essentially the flexibility that metals have in their softer states. Hardened metal has little to no slip.

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For example, auto manufactures used 5160 alloy steel to make truck leaf springs for many years. Chances are an old spring from the salvage yard will be of good quality. However, you can be sure by checking the make model of the vehicle, looking for any numbers stamped into the parts and plugging all of that into google. There isn't a more tragic ending to a project than getting all the way to the end and loosing it because of mistakes in the heat treating process.

Another disadvantage of plain water quenching is associated with what is called the vapor blanket. This is a matrix of vapor bubbles that surround the part as it is cooling. The trapped bubbles can insulate the piece, causing uneven cooling and irregular distribution of stress in the part. Agitating the piece in the quenching medium can break up the vapor blanket and expose the part to "new" cool water.

The only thing more important than the quench medium is the heat itself. The medium in which a part is quenched is of the utmost importance as it will dictate the rate of cooling. Many mediums exits but they basically break down to three categories: oil, water and brine (salt water).

After machining all traces of the cutting fluid should be removed from the surface so that the stainless steel surfaces can self-passivate. Under certain circumstances acid passivation should be considered.

Machines should not be prone to excessive vibration in the machine bed, drives and gear boxes or at the cutting tool or its mountings. Large overhangs of tool shank out of the tool box should be avoided. The distance between the cutting tip and toolbox support should be as short as practicable and the shank cross section as substantial as possible. This can also help in dissipating heat away from the cutting faces. Arbours for supporting barrel milling cutters should be stout and as short as possible. The arbour supports should be as close as possible to the ends of the cutter to provide maximum support.

Steel's versatility comes from its ability to be shaped into different forms, which makes it an excellent material for making tools. Most tools either strike or cut, and since steel is harder than most other materials, it works well for these operations. What happens though, when steel tools are used to cut and strike other pieces of steel?

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Basically, the more carbon that is present in a steel, the harder it is. Hardness is also related to sharpness and brittleness. As the hardness of a steel increases, so does its ability to get and stay sharp. Unfortunately, the harder a steel gets, the more susceptible is will be to cracking and even shattering.

Machining304stainless steel

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Where possible the tool faces should incorporate chip curlers or breakers as austenitic stainless steels are prone to forming long spiralling turnings that can easily wrap around the tool and tool post. These can easily become entangled around the tooling, and are difficult and time consuming to remove. In extreme cases the tool can become jammed by entangled turnings.

We know that martensite forms when steel is cooled rapidly from critical temp to below 617 F. This is called the steel's critical speed. If cooled to fast, some austenite may be present; if cooled too slowly martensite may partly break down into softer constituents. The key is to find the correct info on the material data sheet and / or perform tests before quenching important parts.

Cemented carbides are normally used for machining stainless steels where higher speeds or higher feeds than those that can be produced using HSS are required. Either disposable insert or brazed-on tips, (where lower cutting speeds can be tolerated), can be used, and are composed of either tungsten carbides, or a blend of tungsten and other metal carbides, including titanium, niobium, and chromium. The carbides are bonded with cobalt. The ‘straight’ tungsten carbides grades are used for machining austenitic and duplex stainless steels, and the ‘complex’ carbides are used for machining martensitic and ferritic family grades. Coated carbides have the additional benefit of improved wear resistance and resistance to breakage. Consequently they are capable of higher cutting speeds compared to un-coated carbide tools. The wide range of carbide tools available usually means that machining trials are needed to get the optimum machining characteristics for specific situations.

Either tungsten or molybdenum HSS can be used. These are particularly useful in machining operations involving high feed and low speed machining operations where there are variable cutting edge stresses induced from complex tool shapes. The tungsten types (e.g. T15) can be useful for their good abrasion resistance and red hardness. The molybdenum HSS are more widely used, M42 being useful for applications such as milling cutters where a good combination of hardness and strength are required at lower cutting speeds. M42 has better hardness than grades like the more common M2, but may not be as tough however. If the tools are prone to edge chipping, use a tougher grade, e.g. M2, M10. If tools are burning, use a higher red hardness grade, eg M42, T15. If the tools are wearing, use a more abrasion resistant grade, eg T15.

If you read the preceding Instructables: An Introduction To Heat Treating Carbon Steels, Annealing Carbon Steels, and Normalizing Carbon Steels, then you learned that heat treating is the manipulation of a metal's molecular structure via exposure to specific temperatures. Carbon steel's molecular structure is crystalline and has a grainy appearance. Exposure to heat changes the shape of these crystals. Each crystalline shape exhibits a different set of properties, which can be beneficial in different ways depending on the material's application.

When machining stainless steels it important to ensure that there is no dwell or rubbing caused by machine vibration or tool chatter. Machines must be ‘substantial’ and capable of making the deep cuts needed in machining austenitic stainless steel without slowing down the set feed or surface speeds. Small training or ‘hobbies’ lathes and milling machines intended for machining mild steel, brasses etc. are unlikely to be substantial enough for the successful machining of stainless steels.

Sulphurised, chlorinated or sulpho-chlorinated mineral oils can be used with additions of up to 10% fatty oils for machining non-free machining grades. Paraffin is used to dilute these oils, in oil/paraffin ratios between 1/5 for high speeds and light feed work, to 1/1 for slower speed and heavier feed machining. If excessive wear is being experienced, consider using greater dilutions. If the cutting edge is tending to burn, consider reducing the dilution.

Hardness in steels is directly related to carbon content. Steel is an alloy of iron and carbon. As mentioned in the first Instructable of this series, medium to high carbon steels have a carbon content typically in the 0.30–1.70% carbon by weight range. Low carbon steels do not have enough carbon to be heat treated, and are not relevant to us in this context.

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Milling316 stainless steelspeeds and feeds

It is essential that cutting fluids are used when stainless steels are machined. This is due to the combination effects of the deep cuts and high feed rates needed to overcome the effects of work hardening, and the low thermal conductivity of the austenitic stainless steels, restricting the flow of heat away from the machined faces. Overheating stainless steel surfaces, characterised by the formation of heat tinting colours, during machining can impair corrosion resistance and so must be avoided. If formed, pickling the surface can be used to restore corrosion resistance on the finished part. Overheating can also result in distortion that can be difficult to compensate for or correct.

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Oil is desirable because it provides a fast quench speed though the vapor blanket and then tapers the cooling speed off as the piece enters the lower cooling range. This rate of cooling seems to be ideal for most carbon steels.

Keeping the water clean is also important as contaminants such as algae, soaps, and oils can trap steam and prevent cool water from making contact with the part.

Oil is another widely used quenching medium. It is popular because it slows the cooling rate slightly. There are two general types of quenching oils: "conventional", having no additives, and "fast" which are blended with other ingredients to reduce viscosity.

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Stainless steelmachinability chart

Further information on the selection of cutting fluids for machining is available in the BSSA Training Note No.9 ‘Machining Stainless Steels’

* Hardened carbon steel will typically become harder than a file. Clean the surface with a wire wheel and gently run a file over the materials surface. If it was hardened properly, then the file should glide over the material without making any cuts in the surface. If the file does cut the material, hardness was not achieved. There is a wide variety of carbon steels, each with a slightly different set of properties. It's these differences that give the materials sensitivity to varying temperatures. One steel might harden at 1475°F, while another hardens at 1500°F. There is not much of a difference there, but if the proper heat is not reached, then hardening is at best, partial, and at worst, non existent. Likewise, quenching at temperatures that are too high can cause enough stress to crack, and in extreme cases, tear the material apart.

Steel hardens when it is is rapidly cooled from its critical temperate to below 617 degrees F. This may seem simple, but there are several things to consider in order to make this happen correctly: The quench medium, duration of the cooling, the position of the part when quenching, the quality and thoroughness of the heat, and the shape of the part being hardened.

If I were to rub my finger across my arm, my arm would not slice open. My finger and arm are made from the same material and so there is minimal effect when the two come in contact with one another. So how then does a steel saw blade cut a steel bar?

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The answer lies in the material's hardness. The saw blade is much harder than the bar, thus it is able to be sharpened and maintain that sharpness as it comes into contact with the softer bar. Likewise, if the bar was harder than the saw, then it would be damaged as it tried to cut the bar.

Either high speed steel, (HSS), (wrought or sintered), or cemented carbide tools can be used for machining stainless steels.

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Either mineral oils or water soluble emulsifiable oils can be used. Mineral oils are more suited to severe machining operations with heavy loads at low speeds or where HSS tools are being used. Emulsifiable oils are used for machining at higher speeds with carbide tooling.

These oils are diluted with water and provide better cooling than the paraffin diluted mineral oils. If extreme pressure (EP) emulsifiable oils are used, more severe machining operations can be supported. It is important that dilution is done by adding oil to water, not water to oil so that the correct form of emulsion, with the right lubrication and cooling properties, is formed.

Air / Gas hardening is not widely used, but can be effective for some alloys that require a slow rate of cooling to achieve hardness. Note that hardening an alloy that specifically requires an air quench, is exactly the same thing as normalizing an alloy that does not require an air quench. Confusing? Yep!

Re-sharpening should be done as soon as the quality of the cut has deteriorated. Machine grinding using properly dressed wheels, free from glazing, is preferable to hand grinding to get the necessary accuracy of tool geometry.