Bulletin No. 10. ISO 9001 Registered HEAT TREAT BULLETIN HARDER IS NOT NECESSARILY BETTER

Transcription

1 Bulletin No. 10 ISO 9001 Registered HEAT TREAT BULLETIN HARDER IS NOT NECESSARILY BETTER

2 Harder Is Not Necessarily Better The most common negative trend concerning the heat treatment of tool & die steels recently has been the rash of customer requests for hardnesses above and beyond those that the steels were designed for. Though three of the previous nine East-Lind Heat Treat (E.L.H.T.) bulletins have dealt with recommended hardness ranges for tool steels, this problem of Harder Is Better continues to worsen. Hardness numbers (20 RC, 60 RC, etc.) are not just numbers. Rather, they are an indication of toughness, strength, wearability and brittleness. Unfortunately, hardness, strength, and wearability sit at the same end of the spectrum as brittleness. At the opposite end of the spectrum sits toughness. An ideal, futuristic material would give you both maximum hardness and maximum toughness; however, with today s materials, that is not possible. LOW HARDNESS LOW-STRENGTH MACHINABILITY LOW INTERNAL STRESS TOUGHNESS HIGH HARDNESS HIGH STRENGTH WEARABILITY HIGH INTERNAL STRESS BRITTLENESS Rc Rc Rc In order to produce a sound, high-quality steel product, you must have the proper mix of hardness and toughness to suit your intended application. The following table provides a good rule of thumb for various applications: Desired Condition For Application Maximum Toughness Combination of adequate Strength & Toughness Maximum Strength & Wearability Hardness Range Rc Rc Rc Tool steels classified as water hardening, oil hardening or air hardening should NEVER have a specified final hardness above 61 Rc. The only steels made to perform successfully in this hardness range are the high alloy steels in the high speed category. The reason for this is to ensure that the steel will have sufficient toughness to withstand the final grinding and/or stresses imposed upon it in service. Any time a standard tool steel has a final hardness above 61 RC, it indicates the steel has not been tempered sufficiently to relieve the internal stresses that contribute to high brittleness. Tempering Is Essential To Good Quality Most tool steels sent out to your heat treater for hardening are processed by quenching & tempering. As the name implies, this is a two-step procedure and it is extremely important that your heat treat specifications allow the heat treater to perform BOTH of these operations properly. Though inseparable, let s take a look at each of the quench & temper operations by themselves: QUENCHING: Quenching is the actual hardening operation. During this operation, the steel is heated to its austenitizing temperature (1400 o F o F, depending on the steel type) and held for a certain amount of time. Then the steel is quenched in a suitable media (generally air, oil or water) to generate the maximum hardness.

3 At this point, the steel is in a state of extreme brittleness and high stress. Though very hard, the steel is considered useless for any application. Most steels left in this as-quenched condition will crack spontaneously. To relieve this as-quenched brittleness, it is critical that the steel complete another procedure called tempering (or drawing). TEMPERING: The tempering operation involves heating the as-quenched steel above room temperature to as high as 1100 F. This process does several things to the steel: Tempering decreases hardness. [Desirable or Undesirable] Tempering decreases internal stresses. [Desirable] Tempering decreases brittleness. [Desirable] Tempering increases toughness. [Desirable] We measure tempering in terms of degrees (i.e., temperature). The higher the tempering temperature, the more tempering is done. As tempering temperature increases, hardness decreases (as shown below). As you can see from the graph above, any steel that is tempered at too low a temperature will have a hardness extremely close to the as-quenched, maximum hardness. Thus the steel will be too brittle for subsequent use. The important thing to remember about tempering is this: Tempering Increases Toughness! THE PROBLEM: So what s the problem? If tempering is so darn important to the quality of a hardened steel, then... Why would my heat treater use a tempering temperature that is too low to be effective?! The answer is simple: The tempering temperature is dictated by the customer s hardness specification. The tempering temperature determines the final hardness. When a customer specifies a low hardness, the steel is given a high tempering temperature. When a customer specifies a high hardness, the steel is given a low tempering temperature. When a customer specifies a hardness that is too high for a particular steel, the tempering temperature has to be so low that it is ineffective in relieving brittleness and internal stresses. In a nutshell, even though the hardness complies with the required specifications, the part will likely meet with a premature failure.

4 This is a problem that originates at the engineering or design level. It creates a bad situation for both the tool & die maker and the heat treater. This problem needs to be resolved. So, what is considered too hard? That answer varies with the type and size of the steel. The proper hardness ranges for the various tool steels should be developed using a table of recommended minimum allowable tempering temperatures: Steel Type Minimum Temper Air Quenching Steels: A-2, D F Oil Quenching Steels: O-1, O-2, O-6, L-6, 6150, 4140, 4340, 350 F 52100, Flexor, Carburized Mild Steel Water Quenching Steels: W-H, W-2, 1045, 1060, F Using the above mentioned format, a good rule of thumb for high hardness capability steel is not to request a hardness in excess of 61 RC. High hardness capability steels include, but are not limited to, the following, The 61 Rockwell Rule* A-2 O-1 W-H D-2 O-2 W L-6 O-6 Carburized Mild Steel In addition to the 61 Rockwell Rule," there are a few other rules to be concerned with when other conditions are present. At E.L.H.T., we are proposing the following heat treating procedures for an effective temper of the following grades of steel under the specified conditions: Steel Application Temper Hardness D-Series (D-2, etc.) Low or Non -Stress double 400 F 59/61 Rc D-Series (D-2, etc.) High Pressure or Impacting Load 950 F & 925 F 58/60 Rc Any Steel To Be Nitrided 1000 F Varies D-2 To Be Wire Burned (EDM) 900 F min. 58/60 Rc A-2 To Be Wire Burned (EDM) 900 F min. 56/58 Rc Notice in the above table the tempering temperatures for the two D-series steels. The very hard and more brittle 59/61 Rc is given a double 400 F temper. The much tougher and still very hard 58/60 Rc is given a 950 F followed by a 925 F. That s 550 F more tempering (and toughness) with only a one point drop in hardness! This is a classic example of how tempering increases toughness! Other steels with a moderate hardness capability have maximum hardnesses below 61 RC. The producers of these various tool steels have published the hardnesses and tempering temperature ranges in their steel brochures. The following table lists some of the more common moderate hardness capability steels and their approximate maximum useful hardnesses. Steel Max. Hardness* Steel Max Hardness* /52 Rc /57 Rc /56 Rc /57 Rc /57 Rc Flexor 50/52 Rc /56 Rc /52 Rc B3X 55/57 Rc /56 Rc S-7 56/58 Rc

5 * Please Note: The previously stated maximum recommended hardnesses, including the 61 Rockwell Rule, are based on the heat treatment of fairly small sections of steel. Larger sections, having more mass, quench or cool slower, thus not able to achieve their maximum hardness. This is called the Mass Effect. This is especially true with the water and oil quenching steels. The larger the part, the lower the achievable maximum hardness. To Carburize, Or Not To Carburize These moderate hardness capability steels set the stage for another very common problem: customer requests for final hardnesses above and beyond the as-quenched hardness of the steel. On a daily basis, we see requests for 58/61 Rc (or more) on steels such as 4140 or S-7. These steels, of course, can not achieve these high hardnesses through the normal quench & temper process. The only heat treat method that will accomplish this surface hardness is the carburize & harden process. This process involves heat treating steel in a carbon-rich atmosphere. This introduces a high amount of carbon into the surface of the steel, called case. The depth of this case generally ranges anywhere from.005 to.125 deep. The case is the only portion of the steel that will achieve the high 58/61 Rc. The key to a sound carburize & harden job is a soft, tough core (ideally Rc) beneath the extremely hard, wear-resistant case. The core hardness of carburized steel can be estimated by figuring the hardness of the steel if it were simply quenched & tempered for maximum hardness. For instance, a low-carbon steel (8620, 1018, etc.) would normally quench & temper in the Rc range. However, a moderate hardness capability steel, such as 4140 or S-7, would have a core hardness well above 50 Rc. This is where the problem lies! When the core of a carburized steel is too hard, it lacks the toughness to absorb the stresses generated by the carburizing process. This results in a high failure rate during the heat treat process. If the steel were to survive the heat treating process, it would almost certainly fail in any kind of medium to high stress application. The ideal steel type suitable for carburizing is one that has less than.30% carbon and has little to moderate alloy content. Those steels with over.30% carbon tend to result in too high of a core hardness. Some examples of both good and bad carburizing steels are listed below. Less Than.30% Carbon Steel More Than.30% Carbon Steel L14 But Not Capable of 58/61 Rc CRS S-7 As an example, let s look at S-7 material. S-7 is designed as a shock-resistant, air-hardening steel. It has the highest toughness rating of any tool steel. Steel manufacturers recommend a maximum hardness range of 56/58

6 RC; which requires a 400 o F temper. This temper allows the steel to deliver the maximum shock-resisting characteristics. To arrive at a higher hardness, say 58/60 Rc, you would have to do one of two things; both of which are not recommended. 1. Temper Below 400 o F: This will result in a toughness no better than O-1 material at 60 Rc. At 58.5 Rc, S-7 has a toughness of approximately 100 ft./lbs. using an impact tester. However, tempering at 400 o F results in a 58 Rc with a toughness of 224 ft./lbs. That s more than double the toughness and only a 1/2-point of hardness! 2. Carburize & Harden: This will result in a very high carbon-content surface with a hardness of 58/60 RC The core hardness will be at 56/58 Rc. The surface will have slightly better wearability, but is now brittle as opposed to tough. When combined with the hard core structure, this condition will usually result in chipping in service when the part is subject to repeated impact stresses. Chipping is always an indication of brittleness. Both of the above methods will result in an unstable steel structure not compatible with shock applications, nor able to withstand the stresses encountered in wire burning (EDM) or final grinding. If S-7 is to be used as it was intended (for high and repeated impact), the specified hardness should ALWAYS be 56/58 RC; NEVER any harder. In Summary The specific lessons to be learned and practiced from this bulletin are: 1. Tempering Increases Toughness. Please allow your heat treater to temper properly. The tempering cycle that follows the quench is vital and critical to a high quality finished steel product. 2. The 61 Rockwell Rule. High Hardness Capability steel should never have a hardness over 61 Rc. High speed steels should be used in applications requiring this extremely high hardness. 3. The.30% Carbon Rule. Moderate Hardness Capability steel should never be carburized to achieve hardnesses above the steel s normal capabilities. Carburized steel must have a soft, tough core. 4. Harder Is Not Always Better!! Steels that have a final hardness too close to their as-quenched hardness lack the toughness (too brittle) to perform properly in service. If your company does its own designing and engineering in-house, please make sure copies of this bulletin get to your design & engineering department. If not, please forward copies to your outside sources, or have them contact E.L.H.T. for additional copies or for advice.

7 Written and published by East-Lind Heat Treat, Inc. for exclusive use by its customers. All technical questions regarding this bulletin may be directed to Dale Greer, Quality Manager. To order additional copies of this or other bulletins please contact our office at (248) EAST-LIND HEAT TREAT, INC Dequindre Rd. Madison Heights, MI ATTN.: PLANT MANAGER IMPORTANT INFORMATION