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Our company is using a commercial heat treater and we’re getting good results. Why should we buy a Cress furnace and do the heat treating in house?

This is one question that we hear over and over again and there are arguments to steer the answer in both directions.  We’re going to look at several scenarios so that you can select the correct solution for your company.  Not all heat treaters operate this way, but with the high cost of energy, some are forced to take steps to keep costs low to stay competitive.

Background

You’ve created a tool, a part or series of parts in your machine shop that you need for your operations, or your customer’s requirements.   The designer has chosen A2 tool steel and specifies a hardness of 60 to 62 Rc is needed.  The part or parts could weigh 2 pounds, or 50 pounds and may have taken five hours, five days, or five weeks to make.  Regardless of the time, your company has invested time and money into producing the parts to satisfy a need.  You package the parts up, ship them off to the heat treater and ask them to be heat treated to 60 to 62 Rc.

Scenario Number Two

This is one of my biggest complaints about commercial heat treaters.  The parts arrive, a job ticket is written up.  The job is assigned to a person on the floor who often has no formal training in handling parts or heat treating for that matter.  The company metallurgist sat in their lab and only is called to verify the process to be applied if the leadman, or shop foreman can’t find the process in his private notebook.  The only time the metallurgist gets involved is if the hardness level is incorrect, and you know that the owner of the company will always be in his office concerned with bill paying or sales.

Bottom line of this scenario….. Even if the hardness reads correctly, you have no absolute assurance the times, temperatures, pre-heat, quench or tempering cycles were followed to the prescribed recipe.  Your part quality is at risk.  The life of the part is at risk along with your reputation. Your only hope is to request and pay extra for strip chart recordings of every step of the parts in process.  Even then there are things that can effect the quality of the finished product.

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 in¬tended use.  Anyone using this information or relying on it assumes all risk and any liability arising from their applications and use.

Following a few basic rules can often minimize troublesome distortion that takes place during the steel heat treat process. There are three major areas where distortion can take place, assuming the part is straight prior to heat treatment.

1. The pre-heat step is the first critical distortion prevention step. The pre-heat step, as specified by the steel producer, prepares the metal to transform through the heat treat phase changes, but also reduces the stress introduced by machining, forming, or other processes. Just by programming the controller on your Cress furnace for a 10 minute stop, gives the temperature inside the metal’s mass a chance to catch up to its surface temperature, reducing the tugging and pulling distortion from taking place.

2. At the austenizing temperature, the molecules in steel are ‘in-solution’. That is. they are in a molten state within the shape of the part. That also means they will deform, or sag, to the surface of what they are resting upon. Heat treating is not meant to be done on the Cress hearth plate. That’s an insulation and is not going to allow uniform heat penetration of a part if it’s laying on the hearth plate. A rack should be built that allows even temperature transfer. If the rack becomes distorted, then it needs to be straightened to support the parts in a flat manner.

3. Taking the part from the Cress furnace and transporting it to an air quench rack, or to water or oil quench is next. Because the part is coming from the furnace in the ‘in-solution’ condition, picking the part up has to be done with great care; and if it’s an air hardening metal, needs a flat rack for quenching to prevent sag. If it’s being water or oil quenched, it needs to enter the water or oil straight up and down to avoid cooling one side earlier than the other. Oil or water hard metals may still deform, but it will be less.

If you are heat treating very thin flat parts, often the best way to treat them is to press quench them. How that is accomplished is by removing a single part from austenization and quickly placing it on a flat, heavy plate of steel. Immediately place another heavy plate of steel on top of it. As soon as it loses heat color, proceed with your normal quench method. Remove each piece from the furnace and press quench that same way.

If the part is round or of a peculiar shape, consider making a fixture to hold the part through the heat treat and quench process. For instance, if you have a ¼” diameter rod, 16” long, take a steel pipe with a good schedule wall, drill and tap 3 holes around the pipe every 6 to 8”. Using three bolts in these holes will hold the rod straight in the center of the tube, allowing quenching and a straight part.

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

COPYRIGHT © August 2008, 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.

Decarburization is an oxidizing surface condition caused whenever ferrous (carbon based) metal is heated to temperatures above the visible heat (960^oF) zone, and is exposed to atmosphere. The depth of decarburization penetrates deeper as temperature and time of exposure increases. The surface in this condition has lost carbon composition (de-carbon-ized) and scale (loose flaking surface which resembles the scales on a fish) will also become evident. Hardness in this layer is poor to none, and tool life is definitely and dramatically sacrificed. In fact, if a tool is put into service with any appreciable decarburization, it is guaranteed to fail and can show surface breakdown quickly.

Cress Furnaces are atmospheric box furnaces which means they have air, or heated air in them and there is no protection from decarburization in the furnace. There are several optional methods to protect against decarburization when using a Cress Furnace. They are:

Cress Stainless Steel Inert Atmosphere Tube: You can order your Cress Furnace with a stainless steel inert atmosphere tube used to pump Nitrogen or inert gas into the chamber. It helps, but does not solve the decarb problem because box furnaces are not sealed vessels and are built using fire bricks and contain ceramic element walls that are very porous and full of air and water molecules that cause the surface corrosion.  Purging the chamber does not remove all the air and water molecules and thus decarb will take place. Installing a retort (metal box) inside the chamber and pumping inert gas into the retort works better because the porous fire brick is taken out of the equation; but a thin gray, smutty surface will still prevail because the retort is not air tight.

Stainless Steel Foil, formed by triple folding the edges into an envelope, gives relatively safe atmospheric protection. The disadvantage of SST foil is that the SST is razor sharp, and not particularly inexpensive to use. It provides very good protection, but there is often a grayish surface color that needs to be removed before the tool is used.

Protective Powder is applied to the metal when it is 450^oF which, when it melts, creates a protective barrier around the part. After the heat treat and tempering is completed, the powder may be washed off with hot water for clean up.  Results reported are bright shiny parts, but it’s imperative in all protection methods to remove all traces of cutting fluids, oils or even finger prints.

Protective Paints can be applied to the parts.  After heat treatment is complete, the paint is removed by sand blasting.

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 in¬tended 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

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.

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