Types of Special Processes in Serviacero

To achieve and obtain the best performance of the tooling, once the appropriate steel has been selected, the following recommendations must be followed during the manufacturing processes, otherwise the results could vary considerably in their performance.

Machining must be carried out under good practices for the removal of material by chip removal action; We suggest taking into consideration the recommendations of tool manufacturers, in addition to the optimal machining parameters, given that they could vary according to the metallurgical condition of the steel, type of machine, workpiece clamping, type of cutting tool and machining strategy. 

• If it is dry cutting, we recommend the use of pressurized air for effective burr removal. 

• Avoid overheating the surface, as well as not exerting excess pressure when removing material. 

• Avoid “sharp” corners or edges, acute angles and cutting tool lumps. 

• Avoid marks or irregularities that increase the possibility of tension. 

• Avoid sharp cold cuts, scratches, bumps, etc. 

• Sudden changes in section and/or volume. 

• The surface finish must be as coarse as possible, in order to avoid distortion and generation of fractures during the quenching and tempering heat treatment process.  

• Eliminate surface defects, such as decarburization, cracks, overlaps, etc., all of them inherent imperfections in the hot mechanical forming processes of steel (forging or rolling), for which we suggest based on what is established by international standards, as ASTM A681 (overmaterial for machining per side of 25% greater than the maximum decarburization depth per side).  

Once the coarse machining process has been carried out, if the amount of material removal is “considerable” (more than 25% by weight), it is advisable to perform a stress or tension survey in order to eliminate them derived from mechanical work (machining). ; Doing so greatly reduces the possibility of major dimensional distortions, as well as the possibility of fractures during or after the thermal hardening process (quenching and tempering).  

The temperature for stress relief should be selected once knowing the previous treatment condition of the steel: 

• Steels in annealed condition: the temperature to be used must be ≤ 650°C. 

• Minimum residence time 2 hours, add 60 min / 25 mm excess wall. 

• Pre-tempered steels (hardened or treated): 50°C below the last tempering temperature. 

Reference: ASM Metals HandBook Volume 16- Machining Processes

The correct selection of machining parameters within the processes must be carried out in order to “smooth” the surface and dimensionally approximate the tool to its geometry and working dimensions without neglecting recommendations of the thermal process.

It is advisable to leave an excess of material between 1.5 to 5.0 mm on each side prior to the quenching and tempering heat treatment; When a higher pressure is used during vacuum quenching, the overmaterial must be increased, and this depends on the size of the tool. 

Consider the following practice of these processes:

1. Follow the specific heat treatment instructions for each type of steel and hardness depending on its application and/or characteristic or property to highlight or prevent. 

2. Consider excess material, avoid processing the pieces in final measurements. 

3. It is preferable to perform stress relief (stabilized) after the rough machining operation. 

4. Have the equipment or controls that ensure the correct temperature of the oven and the piece. 

5. It is not recommended to accelerate the thermal process.  

1. The first temper must be immediate in tempera (sudden cooling from austenitizing temperature); Do not allow the piece to reach room temperature for any reason. 

2. Stop cooling between 70 to 80°C. 

3. Maintain at first tempering temperature for a minimum of 2 hours, add 60 min / 25 mm wall excess and subsequent cooling to room temperature. 

4. Evaluate cold hardness (room temperature).  

5. Proceed with second tempering at selected temperature to establish desired hardness. 

6. Minimum residence time 2 hours, add 60 min / 25 mm excess wall. 

7. In high alloy steels or high performance steels, a third temper to remove any traces of retained austenite.  

It is important to consider within the processes that, although the advantages of electroerosion machining facilitate machining operations and/or very complex geometries, the operation itself, due to the temperatures generated during the process, cause superficial changes in the tooling (melting, retempering of the steel and stresses), which can put performance at risk before and during start-up, therefore we recommend the following practice in order to protect and reduce the risk of failures (cracks and fracture).

1. It is recommended that the operation be carried out on previously hardened pieces, in order to generate final pieces with minimal and satisfactory dimensional variations. 

2. In wire cutting, make perforations to join cuts and thus reduce stresses during cutting by eliminating stress concentrators in thick sections. 

3. Finish the operation with fine erosion, several stages, avoid coarse erosion, modify intensity and frequency.  

4. Consult your supplier/brand representative in order to obtain the most appropriate parameters for what is suggested in this section. 

5. Remove the affected surface layer by mechanical polishing or grinding 

6. Perform thermal process relieved of efforts or tensions additional and at the end of the operation by EDM. 

• Reduce the possibility of major distortions, as well as the possibility of fractures during the operation or even before it. 

• Increase in the life time of the tooling. 

• Just the time to carry it out. 

1. Once the mechanical removal of the surface has been done (polishing or grinding), subject it to heating in an oven, it can even be in a conventional oven, and raise the temperature to 50°C Under last tempering temperature with which the hardness of the tool was established. 

2. It is very important to maintain records of the temperatures of the quenching and tempering heat treatment process.  

3. The residence time should be at least 2 hours once the desired temperature has been reached, add 60 min / 25 mm excess wall. 

4. After this time, the piece can be cooled to room temperature. 

5. Finish tool to final dimensions and desired surface finish. 

Reference: ASM Metals HandBook Volume 16- Machining Processes. 

Every time the tools have worked, it must be considered that the tools, by their nature, store work energy called “residual stresses or tensions”, and that, during runs or mechanical work, the work energy is not always dissipated by the tool. Therefore, the stored energy generates failures, both surface (thermal fatigue) and internal (crack generators), that is, if the residual work stresses are not eliminated, they will limit the toughness, detracting from the performance of the tool. .  

It is necessary to ensure that the tool is in condition to continue with cycles or production runs in the processes, that is, that at least superficially it does not present damage, faults that put at risk the operator, the operation, the equipment and the product, ensure that it is free of cracks, otherwise correct the damage.

• Verify and make sure you know the temperature used in tempering at which the use hardness was established and subject it to a temperature of 50°C below this. 

• Minimum residence time 2 hours, add 60 min / 25 mm excess wall. 

Nota: If the tooling has superficial cracks (thermal fatigue), it is necessary to eliminate them before performing the stress survey, otherwise this action will accelerate the generation of fractures. 

• After this time, the piece can be cooled to room temperature. 

• Another practice to eliminate residual stresses or tensions is with mechanical vibratory work, the process is slow and is used when there are no ovens for the purpose. 

Every time the tools have worked, and after maintenance, the tools must be kept clean and dry, since they are susceptible to rust and corrosion, therefore, they must be stored in dry places and avoid being exposed to humid conditions.  

Proper and thorough process management of tool life must be considered an integral part of the daily operation of tooling.

Although emergency situations always occur, scheduled maintenance drastically reduces both the occurrence of failures and accidental events. 

Reference: ASM Metals HandBook Volume 16- Machining Processes

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