FAQ'S!

Q. What causes retained austenite in tool steels?

ANS. » Retained austenite is created as a result of untransformed austenite. It is usually indicated by a hardness check after quenching, when it is seen that the as quenched hardness is approximately 4 to 5 Rockwell C scale hardness points low/. This is usually (but not always) indicative of the presence of retained austenite.

Q. What is Heat Checking?

ANS. » Tools that are made from Hot Work Die Steels, are not only subjected to high service temperatures, but to high temperature fluctuations during operation.This will cause surface cracking on the work surface because of the high temperature fluctuations. The cracks grow in a reticular fashion, not only in length, but in depth. The crack is also being subjected to both decarburization and to oxidation during the cracking and repeated temperature fluctuations. The only thing that can be done to reduce the risk of heat check is to keep the shot sleeve well cooled and to control the temperature of the molten aluminum entering the sleeve. The aluminum is hitting the sleeve in the molten condition, and at an very elevated temperature which only agrevates the condition. 1) Control the aluminum temperature. 2) Reduce the aluminum temperature. 3) Nitride the sleeve.

Q. How do we identify surface decarburization?

ANS. » You can identify surface decarburization by hardness testing. If the surface hardness is lower than the expected as quenched hardness, then grind into the steel and check the hardness again. If the hardness has increased, then there is a strong possibility that decarb is present. However, it could also be retained austenite. You can also check it by cutting a sample through the suspected test coupon, pre-grinding, polishing, and then etching with a 3% to 5% Nital etchant and immerse for approximatly 5 seconds. Examine under a microscope at say 400X to 800X and you will see the surface decarb etched out white at the surface for the depth of the decarb.

Q. Is it always necessary to pre heat tool steels when austenitizing?

ANS. » It is most advisable to preheat tool steels prior to austenitizing. Remember that distortion is not always caused by quenching. It can be caused by the steel stress relieveing itself as it is ramping up to its appropriate austenitizing temperature. Induced stresses are best relieved by the application of heat. And heat is applied when ramping up to temperature. In addition, pre-heating gives the tool steel an opportunity to reduce the risk of themal shock. There is no formulation as to how fast or how slow to heat up to temperature. My personal recommendation is to heat up at a rate that is both appropriate and economical for the particular steel, in relation to its composition and cross sectional area/geometrical size changes.

Q. What is meant by the term 'secondary hardening'?

ANS. » Secondary hardening is a term that is usually (but not in all cases) used in the heat treatment of tool steels. If you consider the higher alloyed tool steels such as High Speed Steels (HSS) Cold work tool steels (D series) Air hardening( A series) then the secondary hardening phenomena is very apparant. After austenitize and quench, the steel is in the martentsitic condition and it is necessary to temper. If the tempering temperature is kept at a low temp(400 F to say 600F), there will be a reduction in hardness due to a conversion of un-tempered martensite to tempered martensite. If one then tempers at an elevated temperature (say 1000F), then secondary hardening will begin. This means that the alloy elements that are in solid solution, begin to react with the carbon present in the steel to form carbide precipitates of those elements. The carbides precipitate out of solution and can be readily seen microscopically. The net result is that the hardness value will begin to increase. It will then reach a maximum hardness in relation to tempering temperature, and will then begin to reduce its hardness value.

Q. How can we keep A2 tool steel bright and clean when air hardening, without using a vacuum furnace for surface cleanliness?

ANS. » The reason for the A2 becoming disclored/scaled is simply because of an oxygen attack at the surface of the steel if heat treating with out any surface protection. It is said 'use nitrogen'. All that nitrogen will do is to reduce the risk of surface oxide forming. However it will not stop decarburization. In order to protect the steel surface, and when you are preparing to wrap the steel in stainless steel form, simply wipe a smear of oil onto the steel surface and onto the inside face of the stainless steel foil. Seal the foil and proceed as normal with your heat treatment of the A2. When the steel is ramping up to temperature, the oil inside the 'package' will ignite because of the presence of oxygen and heat. The oxygen will be consumed and transformed to carbon monoxide and some carbon dioxide. There will be no oxygen to attack the surface and surface of the steel will remain clean.

Q. How many thermocouples are necessary to do a temperature uniformity survey?

ANS. » The number of thermocouple to accomplish a temperature uniformity survey will depend on the configuration of the furnace, A horizontal box furnace will require a minimum of nine thermocouples. A vertical air circulation furnace would require an minimum of fifteen themocouples. Then of course one needs to consider is the furnace a batch type or a continuous type of furnace

Q. Why is it necessary to pre-heat a tool steel prior to reaching the austenitizing temperature?

ANS. » Most tool steels are very sensitive to thermal shock. If the tool is placed directly into a furnace at the austenitize temperature, there is a great risk of cracking and most certainly of distortion. On the ramp up to austenitizie temperature, the steel will also be stress relieving itself of induced residual stresses from machining. If the tool steel has differing cross sectional geometries, there will also be differential heating rates. On that basis, the steel will move out of the geometrical tolerance. Further, because of the geometrical cross section thickness variations, the point of geometrical sectional size change will act as a stress raiser. Therefore, ramp the steel up to temperature with holding platforms in the ramp up procedure to allow the cross sectional variations to equalize.

Q. What is the probable cause of the cracking?

ANS. » Remember that EDM is melting the immediate surface of the die material in order to remove stock to accomplish the die form. The generated spark, from both the elctrode and the die is melting the die surface. The melted material is flushed away by a 'dielctric' cooling fluid. It should be noted that there are two methods of EDM , which are Wire EDM and Sink EDM. The same risk is with both methods of EDM. There is also a heat sink that occurs as a result of the surface melting, that traverses back into the surface of the steel. Think of the Iron Carbon Equilibrium diagram, in order to melt the steel the phase changes in the surface go from BCC to FCC to BCC (liquidus). The heat that is migrating back thinto the body of the steel is also transforming the body of the surface of the die. The dielectric fluid that is acting as a process coolant, now acts as a quernch medium thus forming untempered martensite on the die surface, This is usually followed by the strong probability of surface cracking. Therefore temper the die immediatly after EDM work to reduce the risk of crack propagation

Q. What are the causes of distortion?

ANS. » The causes of distortion are many and numerous. One aspect of distortion is, that the distortion will only manifest itelf at the heat treatment procedure. As soon as heat is applied to steel/metal, any residual stresses that is present will manifest itself in the form of distortion resulting from stress relieving. There are many reasons why distortion occurs, such as from induced machining streses, from variations in the machining procedure, non metallic inclusions offering differential rates of expansion. Distortion can also be identified as a result of volumetric growth, originating from surface treatments. When you change the composition of a metallic surface by diffusion techniques, there will be growth due to a surface volume change.This applies to nitriding, carburizing etc. There will also be a size change in any metal due to phase changes that occur as a result of the application of heat.In steelo that would be the changes from ferrite to austenite, and to martensite.

Q. What is the difference between H11 and H13 Hot Work die steels?

ANS.» There is only a very slight difference in the two basic chemistry's. The difference is in the vanadium content. H11 is at 0.40% and H13 is at 1.00% The significance of this is that with H13, the austenitize temperature is slightly higher, the MS transformation point is higher with H13 (MS =630F approx, and H11 = 510F approx) The time to transformation of Martensite is very similar in both steels. The as quenched hardness value with H13 is slightly lower than H11. However care must be taken with both steel on austenitizing temperature selection and soak time. If extended soak times are given, then there will be grain coursening of the austenite grain, which in turn will allow more carbon into solution as the alloy carbides begin to disolve.(Chromium, vanadium, and molybdenum) This can lead to premature failure of the die. Keep the austenitize temperature on the low side of the range, do not over soak, and temper immediatly.

Q. What is meant by quench cracking?

ANS.» Quench cracking is generally caused by when a workpiece is heated to an elevated austenitizing temperature, followed by quenching (or rapid cooling) The commonly experianced quench cracking occurs at the surface of the workpiece as a result of the phase transformation for austenite to martensite.The most common quench cracking will initiate in an intergranular manner, along the grain boundries. It is most im;portannt to temper immediatly after quenching for two reasons. The first reason being that there is a very serious risk of cracking occuring due to the fresh martensite being left in an unstable condition. Tempering will induce that stability to form tempered martensite. The secon reason is, that the longer the steel is left after quenching before tempering occurs, the more difficult it will be (in terms of tempering time) to accomplish the required, as tempered hardness Quench cracking is generally caused by when a workpiece is heated to an elevated austenitizing temperature, followed by quenching (or rapid cooling) The commonly experianced quench cracking occurs at the surface of the workpiece as a result of the phase transformation for austenite to martensite.The most common quench cracking will initiate in an intergranular manner, along the grain boundries. It is most im;portannt to temper immediatly after quenching for two reasons. The first reason being that there is a very serious risk of cracking occuring due to the fresh martensite being left in an unstable condition. Tempering will induce that stability to form tempered martensite. The secon reason is, that the longer the steel is left after quenching before tempering occurs, the more difficult it will be (in terms of tempering time) to accomplish the required, as tempered hardness.

Q. What is the austenitizing temperature for the Hot Work die steels?

ANS.» This is a very broad and wide ranging question, as there are three principle groups of Hot Work die steels. The first being the Chromium group(H10 up to H19 grade)( the most widely used being H13) followed by the Tungsten group (h51 up to H 26) followed by the Molybdenum group (H42) All of the hot work steels have a medium carbon content ranging from 0.35 up to a maximum of 0.55%.This means that the as quenched hardness will not be of a high hardness value. All of these steels will require high austenitizing temperatures ranging from 1760F up to 2150F (Tungsten grades) The H13 will austenitize at a minimum temperature of 1850F. Selection of the appropriate steel will be dependent on the application and the work enviroment. Another feature of each of the groups of hot work steels is that they will all exhibit the secondary hardening effect at the tempering operation. A correctly heat treated hot work steel should display(microstructure) a structure of finely dispersed fine carbides in the matrix of tempered martensite. This will produce a good hot hardness for the particular steel grade.

Q. What is residual stress and how is it caused?

ANS.» Residual stress is caused in the first instance by machining or mechanically shaping the metal (albeit steel or non-ferrous metals). The act of machining twisting, bending, rolling or forging, will induce the stress deformation pattern into the surface of the metal. The only effective method to remove or neutralize the induced stress is by the application of heat. Further, phase change from Ferrite to Austenite, to Martensite will also induce residual stress. This will also be seen as a size change due to the volumetric size change of each of the creatred phase changes