EPS Heat Treat Headlines

Water in quenching oils can be very dangerous, and even more so when martempering using oils at temperatures above 212oF. Where does water come from? Generally it is from condensation due to the heating and cooling cycles the oil goes through. The colder the climate the worse the problem can be if the quench tank is sitting on a cold floor. Water in oil generally collects and settles to the bottom of the quench tank, but can also become emulsified in a circulating tank. If the oil is stagnant, or not circulated, the red hot parts in the bottom of the quench basket can be quenched in water, which in turn can cause non-uniform hardness, improper as-quenched hardness of the parts, and very often cracks will also occur from the faster quench. If the water level is high enough that red-hot parts cause the water to boil, an explosion can take place from the large amounts of steam that is formed.

A larger amount of water can exist with less chance of explosion, in a tank with circulating oil because the steam cannot form; however, it can create foam. However, foam is also a very dangerous fire hazard since oil foam can catch on fire very easily.

It is generally a good practice to check your quench tank every 6 months to make sure your water level is under control. The best method is to pump the quench oil out and observe the mixture at the bottom of the quench tank. You will be looking for water, muck, dirt or any contamination which, if found, should be removed and disposed of properly. Muck and sludge in the bottom of the tank also can affect the effectiveness of the hardenability of the oil. If the oil becomes emulsified, it cannot be salvaged and must be replaced.

Another quick method is to attach a long handle to a shallow container which you lower to the bottom of your quench tank. It must go deeper than the bottom of your quench basket, and by removing it slowly and carefully, you can bring a sample of the bottom of your tank to the surface.

COPYRIGHT © April 2007, by Advisor In Metals

ALL RIGHTS RESERVED. No part of this publication may be reproduced, transmitted or copied without prior written permission of the author and publisher.

Years ago mills supplied alloy and tool steels with a spherodized annealed structure. Due to economic constraints most, if not all, mills have short cut that process. They still anneal, but it isn’t a sphereodize annealed any longer. Today’s annealing yields a pearlitic microstructure with an increased hardness. The result, or effect, is that the metals generally don’t machine as well as they used to a few years back and surface finishes also suffer. In applications that require machining in tough, tight tolerances, difficult design areas, it often would make sense to properly anneal the material either before machining begins or, better still, once the part is roughed out to a near net shape. It is better after being roughed out because it also eliminates most of the stresses and reduces some of the deformation.

To properly spherodize anneal any steel, use the annealing temperature as stated by the manufacture of your steel, or refer to the ASM Standards. The best method is to put the steel in your Cress furnace and heat the steel to the annealing temperature. Then lower the temperature slowly (preferably 25 degrees per hour) to 900 F and then shut off the furnace. Do not open the furnace at all, and allow the furnace to return to room temperature. The process will take 22 to 24 hours but will produce a totally uniform grain structure which contain small, neat and orderly globular shaped carbides in a smooth flowing ferritic matrix.

Low carbon steels are not normally spherodized for machining since they become soft and gummy, but can be spherodized when increased ductility is desired for bending or forming parts.

COPYRIGHT © May 2007, by Advisor In Metals

ALL RIGHTS RESERVED. No part of this publication may be reproduced, transmitted or copied without prior written permission of the author and publisher.

Stress relief heat treatment is used to remove stress induced in metals from various manufacturing methods. Some of these methods include: milling, turning, welding, bending, heating, cooling, shearing, forging, sawing, grinding, not to mention the steel making processes that leave the metal full of residual stresses. These stresses can cause harmful distortion, brittle fracture, and stress corrosion cracking near welds and within some grades of metal.

Removing residual stress is a time/temperature related event with a very controlled cooling cycle using your Cress Furnace. If not carried out correctly, new residual stresses can be produced that will result in greater stresses than the part had originally. To remove stresses it is recommended that you consult the mill literature for the grade of metal to determine what the Ac1 temperature is for your application. There are several methods to remove stresses successfully, but the most commonly used method is heat treat stress relief. The main criteria to use for choosing the correct temperature is to heat below the lower austenizing temperature (Ac1). Decarburization will take place above 960oF, so protect the surfaces if the surface is not going to be removed by machining.

Thus, if we have a 4140 steel part, (4140 has a 1380oF Ac1) that we want to stress relieve, we could place it in our Cress furnace, take it up to 1100oF and soak it for 6 hours, followed by controlled cooling at a rate of lowering it at 50 to 75oF per hour, in a closed furnace, to below 400oF, at which point it can be removed from the furnace, and it would be stress relieved. However, if you heated it to 1300oF, you can soak it for just an hour, followed by the controlled cooling and also stress relieve the part.

Yes, by PACK CARBURIZING. First, let me clarify. There are several ways to carburize low carbon steels, but in an open atmosphere, box type furnaces like Cress furnaces, there are only two ways. One way is to use a carburizing compound. But buyer beware. Many carburizing compounds contain oxidation chemicals that literally eat other chemical elements in stainless steel. And, using these compounds, even just one time, saturates the porous fire brick to a point that if you ever desire to use stainless foil for decarb protection, the chemicals will literally eat holes in the stainless foil.

Now on to pack hardening: Pack hardening is a two step process that works well for low carbon steel carburization. First, you must have a completely sealable steel box. This is often a bolted together box that fits in your Cress Furnace that has sealing insulation rope between sides and ends of the box. A heavy wall pipe with sealable, removable end caps works well.

The box needs to be filled with any good carbon rich material, such as, cast iron turnings or chips, or something like powdered bone meal. When filling the box, parts can be introduced in layers, with no parts touching one another, then more layers upon layers until the box is completely full. The end of the box must be sealed using insulation sealing rope.

Next, the box can be placed in the furnace and heated as required for the grade of steel being treated, usually in the 1700 to 1900oF range. The whole box must come up to temperature, then soaked for 16 to 48 hours, which is dictated by the depth of case needed; the longer it is soaked, the deeper the case will be. The carbon essentially gets absorbed on all exposed surfaces of the parts during the soak process. Once the soak time is complete, the box can be removed from the furnace and cooled. Once cooled to room temperature, the box is opened and the parts can be removed from the box and are then ready to be put through the second step which is the heat treating process as required for the grade of steel being used.

The packing material can be reused several times before it needs to be refreshed for more available carbon.

COPYRIGHT © May 2007, by Advisor In Metals

ALL RIGHTS RESERVED. No part of this publication may be reproduced, transmitted or copied without prior written permission of the author and publisher.

I’m not sure there is one pat answer to that question because there are some heat treating processes which are not always economically feasible. But first, let’s consider what you might be getting for your money when you use a commercial heat treater.

Commercial heat treaters are in business for one purpose… Making Money. Most commercial heat treaters have large or medium size furnaces and they must have a sufficient volume of metal in that furnace to justify turning it on. Thus, if you send them 2 lbs or 100 lbs of A2 tool steel, they may not be able to justify running the furnace until they have 300-400 pounds collected. So delivery time becomes an issue.

But worst yet, is if the commercial heat treater is not quite ethical, they may load your A2 tools, which require 1775F, in with some S7 that requires 1725F and D2 or H13 which requires 1850F and heat treat all at 1825F. What that does is over cook your A2 and S7, resulting in a low hardness and excessive retained austenite, which they fix by freezing in a mechanical freezer to –150F. That transforms enough austenite to martensite to get by and raises the hardness back to an acceptable level. But even though the hardness reads fine, the grain structure is not correct and the tool’s life is affected. The D2 and H13, on the other hand, are not quite cooked enough, which means you are missing all the carbides the steel could offer because they did not get into full solution during the soak. Again, the deep cold will bump the hardness up to acceptable levels but, again, your tools will not wear as long as they should and may even chip easily.

So far we haven’t even discussed the various sizes of the parts in the mix. A2, D2, H13 and S7, plus other air hard steels, require a soak at austenizing temperatures of 1 hour per inch of cross section. So, if the load contains small parts and larger parts, they must heat treat based on the size of the larger parts in order to get any hardness at all. This over cooks the smaller shape parts and again using deep cold they gloss over the problem and fool the customer.

Next is tempering. Tempering should be started once the metal is quenched to below 150oF, and never allowed to reach room temperature and if it does absolutely should it never be allowed to sit at room temperature for more than 2 hours before the tempering begins. This is something that happens very commonly at many commercial treaters. It is so important that from a mill stand point a piece of steel that has been allowed to sit longer than 2 hours at room temp should be annealed and re-heat treated because that much damage; that much life has been lost. And, all steels require a minimum temper of 2 hours per inch of cross section. This does happen in some reputable commercial shops, but, again, is a major value concern if it doesn’t. One hour of soak is not sufficient because the entire mass does not get to the tempering temperature.

Please remember, most commercial heat treat companies hire lower cost workers to actually handle your parts during the processes, normally with little or no supervision. Having visited over 100 commercial heat treat shops in New England, NY and NJ, I’ve only found a small hand full that actually have their act together and the life of your tools and parts are in their hands.

No, I’m not against all commercial heat treaters. There are good ones out there, but often they are hard to find without a lot of evaluation. And there are times that you will need them when specialty processes can’t be done in smaller in-house heat treat equipment. But it’s wise to be careful when choosing or specifying the use of a commercial house. A machinist can work for days to make a tool that can be ruined in a couple hours by a poorly controlled process.

With a Cress Heat Treat Furnace you can take control of your own destiny as far as heat treatment and turn around time is concerned.

COPYRIGHT © September 2007, by Advisor In Metals

ALL RIGHTS RESERVED. No part of this publication may be reproduced, transmitted or copied without prior written permission of the author and publisher.

Question 6: I have an A2 part that has a main body that is 2” cubed, but it has a part that sticks out another 2” but is only ¼” thick on one side. The print calls for a 60 Rc but I can’t figure out how to make it work. How do I heat treat it and not over cook or under cook it?

Actually it’s not that difficult. Before you wrap it in SST foil and put it in your Cress Furnace, clamp a block onto the ¼” section and fool it into thinking it’s a 2” thick section. When you heat treat the part, it is all uniformly soaked as though it is a 2” thick block that is 4” long.

Question 7: My Cress furnace is set up for the 2400F higher temperature. Can I heat treat high speed steels and powdered steels?

Yes, a Cress furnace set up for 2400F can heat treat high speed steels and powder metal tooling. But first, I generally don’t recommend heat treating these metals if you only do them occasionally. The reason I take this route is because high speed steels have extremely short austenizing soak times. Most all of them need to be soaked for 5 minutes maximum per inch of thickness once they reach austenization temperature. So, over soaking them in the length of time they are in the furnace is extremely critical, and that soak time literally comes down to seconds. In fact, every second in excess time takes away longevity of the tool’s life. For that reason, I generally suggest sending the tools to a good commercial house that works on high speed tools everyday.

But, if you have more than just an occasional piece to do and your operator is able to do some scrap pieces and record how he does them, then by all means, you can heat treat these steels very successfully.

One further word of caution. When the parts are removed from the austenizing temperature, if you are using the oil quench method, they must be quenched in the oil immediately, but the door to the furnace should also be closed as fast as possible. A furnace at 2400oF receiving an extended incoming flow of room temperature air receives a heavy thermal shock, which over time will effect the longevity of the ceramic and fire brick liner.

Question 8: Cress offers a Draw furnace but I can’t afford to spend the money. How important is it to have a draw furnace?

When a piece of metal comes out of the furnace and goes through the quench, whether by air, oil or water quenching, and the temperature drops below 150oF it should be placed in a pre-heated tempering oven or the grain structure will be effected. That will cause premature tool failure. If the furnace is at 1750oF, it takes a long time for the heat to dissipate and to get it down to 400oF for tempering. Opening the furnace and force cooling will cut short the life of the fire brick and ceramic plus, may have an affect on the elements. Thus it makes sense to have a draw furnace that can be pre-heated and ready to complete the process.

DISCLAIMER OF LIABILITY
The material presented in this article is intended for general educational information only. It should not be used for a specific application without careful analysis and study of the intended use. Anyone using this information or relying on it assumes all risk and any liability arising from their applications and use.

COPYRIGHT © April 2007, by Advisor In Metals

The author, Bill Bryson, Advisor In Metals has had numerous years and extensive experience in the heat treating of tool steels. He has conducted over 250 seminars to leading companies in the U.S. to train their tool makers and engineers on proper steel selection and heat treatment practices. He is also the author of the book called “HEAT TREATMENT, SELECTION AND APPLICATION OF TOOL STEELS” published by Hanser-Gardner Publications.

If you want practical information on the heat treatment process in understandable everyday language, inquiry to: Advisor In Metals or via e-mail at thegateway@metrocast.net. Information about the book or seminars is available on line at: Advisor In Metals

Question 4: How do I know if my Cress furnace temperature controller is calibrated correctly?

Simple! Unless you check it, you don’t know. For that reason I always suggest customers purchase the Cress pyrometer with their furnace. It does several things.

First, with the thermocouple supplied, setting directly on the parts being heat treated, the operator will know for certain when the parts are at austenizing temperature instead of relying on ambient air temperature inside the chamber. That means less opportunity of over cooking, or under cooking parts.

Second, it also acts as a check against the temperature recorder readings in the event there is a wide difference, which would indicate either one or the other is out of calibration.

Question 5: I don’t always have time to temper my parts so I put them into my Cress furnace and temper them the next day when I get to work. Is this OK?

NO!!!! After quenching a part, it is loaded with freshly created Martensite. Martensite is a fine grain structure, but in the un-tempered condition it is extremely unstable. It is so unstable it can literally explode and send razor sharp shrapnel like pieces 25’ in all directions. It can be triggered by performing a hardness test, heating it, chilling it, vibrations and especially impact.

In addition, it is a fact that an un-tempered part that sits at room temperature for over 2 hours loses so much in good attributes, that economically it would be best to anneal the part and re-start the heat treat process from the beginning. The un-tempered part will still retain its hardness and will temper and on the surface look just fine; but the fine grain structure will have gotten course, and will refuse to transform additional Martensite as usually happens during the tempering process.

Solution: Never start heat treating unless you have time to complete the entire process. If you absolutely cannot finish, then quench the part to below 150oF, then put it back in your Cress furnace (make sure management, the fire inspector and your insurance company approve of an unattended furnace in operation) and raise the temperature back above 200oF and you may leave it there till the next morning or till the following month, and it will temper and form the proper martensite structure. Just avoid letting it sit at room temperature. In fact, if permitted, you can temper the parts at the correct tempering temperature instead of the over 200oF temperature. Remember, you can under-temper a part, but you can never, ever over-temper a part by holding too long. Tempering temperatures will not over cook a part.

DISCLAIMER OF LIABILITY
The material presented in this article is intended for general educational information only. It should not be used for a specific application without careful analysis and study of the intended use. Anyone using this information or relying on it assumes all risk and any liability arising from their applications and use.

COPYRIGHT © April 2007, by Advisor In Metals

The author, Bill Bryson, Advisor In Metals has had numerous years and extensive experience in the heat treating of tool steels. He has conducted over 250 seminars to leading companies in the U.S. to train their tool makers and engineers on proper steel selection and heat treatment practices. He is also the author of the book called “HEAT TREATMENT, SELECTION AND APPLICATION OF TOOL STEELS” published by Hanser-Gardner Publications.

If you want practical information on the heat treatment process in understandable everyday language, inquiry to: Advisor In Metals or via e-mail at thegateway@metrocast.net. Information about the book or seminars is available on line at: Advisor In Metals

As a seller of Cress Heat Treat Furnaces we often get questions from end users regarding heat treating. We have taken the time to answer of few of those questions here, and will be posting a few more in the coming weeks so be sure to check back.

Question 1: I heat treated a piece of D2 in my Cress furnace and the part physically shrunk. Now it’s too small to use. Why?

Answer: The only reason a part will shrink in size after being heat treated is because it was over cooked. D2 is meant to be soaked at 1850F for 1 hour per inch of cross section. In many cases people will get busy and often don’t get back to the furnace when the time has expired. As little as 5 or 10 minutes too long and you will experience over cooked parts.

Solution: Use the auxiliary contacts on the temperature controller of your Cress furnace to light a light or ring a bell to make sure you don’t over soak your parts.

If you lose size in a part from over cooking during the heat treat process, the part can be easily fixed by packing it down in dry ice or liquid nitrogen for 8 hours. This will cause the out-of-phase retained austenite to transform and the size will snap back to its original size.

Question 2: I heat treated a part made from A2 and when I put it on the surface grinder, it didn’t have much attracting magnetism. What causes this?

Answer: Essentially, it’s the same over cooking as we described in question 1, only the part may not have shrunk. When steel reaches its austenizing temperature, it’s referred to as being in-solution. When that happens, the crystal structure is between stages and is essentially free floating with a loss of magnetism. If over cooked by too long, or too high a temperature, it never snaps back into phase and thus, magnetism is lost.

Solution:Again packing on dry ice will solve the loss of magnetism.

Question 3: Why do heat treat recipes call for a pre-heat before going to the austenizing temperature? Is it really necessary?

Answer:Yes, it is necessary. It does two things. It gives the temperature on the interior of the part being heat treated time to equalize with the surface temperature. This helps relieve stresses and reduce distortion. But, it’s also essential to get the chemistry ready to transform into a proper austenite structure with better opportunity to form fine grained martensite after the quench.

DISCLAIMER OF LIABILITY
The material presented in this article is intended for general educational information only. It should not be used for a specific application without careful analysis and study of the intended use. Anyone using this information or relying on it assumes all risk and any liability arising from their applications and use.

COPYRIGHT © April 2007, by Advisor In Metals

The author, Bill Bryson, Advisor In Metals has had numerous years and extensive experience in the heat treating of tool steels. He has conducted over 250 seminars to leading companies in the U.S. to train their tool makers and engineers on proper steel selection and heat treatment practices. He is also the author of the book called “HEAT TREATMENT, SELECTION AND APPLICATION OF TOOL STEELS” published by Hanser-Gardner Publications.

If you want practical information on the heat treatment process in understandable everyday language, inquiry to: Advisor In Metals or via e-mail at thegateway@metrocast.net. Information about the book or seminars is available on line at: Advisor In Metals

Engineering handbooks point out that the classification of steels comes under the authority of the Society of Automotive Engineers (S.A.E.) or American Iron and Steel Institute (A.I.S.I.) for its coding system. The system of coding was originated in the early 1940’s and it was done with the expressed intention of giving engineers, designers, draftsmen, or heat treaters specific information on the types and grades of steel available. The system is very effective for steels using the A.I.S.I. and S.A.E. standards.

The S.A.E. or A.I.S.I. classifications contains 4 to 5 numeric characters with additional alpha characters added to designate special characteristics of the steel. Here is the coding system definition. (Examples: 1018, 12L14, 4140, 41L40 )

The very first number depicts a general category grouping of steels. That is:

The second number gives indication if there are elements present that effect attributes of the steel. The last two digits (three digits for a few grades) represents the actual nominal percentage of carbon content present.

Take for instance 1018 steel. The zero in the 10XX indicates there are no major secondary elements present such as sulfur. Sulfur in steel increases machinability, but all free machining agents, such as sulfur, lead, calcium, etc., are in essence dirt, or elements taken directly from the earth. Such free machining elements improve the machinability but do not homogenize in the steel making process and can cause pockets, stringers or other faults that can affect some applications.

The last two characters represent the carbon content of the steel. For instance, 1018 is a basic carbon steel, with no added alloying elements, and contains .18 % of carbon. The actual standards will show a carbon range of .15/.20%. During the steel making process the carbon and alloy contents cannot be controlled to a specific percentage and thus the percentage shown is stated as the nominal.

Now let’s us examine what takes place if you see an 11XX number. The first ‘1’ indicates it to is also a simple carbon steel, the second ‘1’ indicates the basic analysis has been modified. In steel bearing the ‘11’ designation examining the chemistry tells us sulfur has been added to improve machining. These steels are typically used in automatic screw machine operations, or often when threading needs to be done. (thus any grade carrying ‘11’ refers to re-sulfurized steels. Re-sulfurized means the sulfur was added to the crucible at the end of the heat, prior to pouring into an ingot, which prevents it from being burning away. Familiar grades such as 1113, 1117, 1141 are re-sulfurized grades.)

There are some gaps in the numbering system such as 2XXX, 3XXX and 7XXX. At one time there were steels in some of these categories but were not popular enough to continue in manufacturing and were retired.

Often you will see an alpha character has been added in between the code groups, such as: 11L17, 11L41, 12L14 or a 50B40. The ‘L’ designates a lead addition. The lead like sulfur improves machinability and if combined with an already re-sulfurized grade, improves it that much more. The letter B stands for Boron which is on occasion added to low carbon steels to aid in increasing the hardness of the steel.

Steels with the letter ‘H’, which simply means ‘hardenability’, added to the end of the grade designation, signifies the steel is guaranteed to be capable of hardening to a specific depth.

We already briefly looked at the 1XXX grade classification and explained 2XXX, 3XXX and 7XXX are not being used, now here are the remaining classifications of steel categories.

4XXX Series are Chromium Molybdenum Steel.

Examples:

• 41XX Chromium Molybdenum (4150)
• 43XX Chromium Nickel Molybdenum (4340)
• 48XX Nickel Molybdenum (4815)

5XXX Series are Chromium Steel Low Chromium.

Examples:

• 51XX High Chromium (52100)
• 6XXX Series are Chromium Vanadium Steel
• 8XXX Series are Nickel-Chromium-Molybdenum Steel
• 9XXX Series are Silicon Manganese Steel

DISCLAIMER OF LIABILITY
The material presented in this article is intended for general educational information only. It should not be used for a specific application without careful analysis and study of the intended use. Anyone using this information or relying on it assumes all risk and any liability arising from their applications and use.

COPYRIGHT © April 2007, by Advisor In Metals

The author, Bill Bryson, Advisor In Metals has had numerous years and extensive experience in the heat treating of tool steels. He has conducted over 250 seminars to leading companies in the U.S. to train their tool makers and engineers on proper steel selection and heat treatment practices. He is also the author of the book called “HEAT TREATMENT, SELECTION AND APPLICATION OF TOOL STEELS” published by Hanser-Gardner Publications.

If you want practical information on the heat treatment process in understandable everyday language, inquiry to: Advisor In Metals or via e-mail at thegateway@metrocast.net. Information about the book or seminars is available on line at: Advisor In Metals

Originally Published on EPS Ovens and Heat Treat Furnace

ALL RIGHTS RESERVED. No part of this publication may be reproduced, transmitted or copied without prior written permission of the author and publisher.