NAK55 Steel

Machines up to 50% faster than P20 to a superior surface with negligible dimensional change. Positive rake cutters with concave, chip breaking inserts work best, and last up to twice as long as with P20 mold steels.

The soft recast layer (approximately 32 HRc) is much thinner and is easier to remove than steels with a recast layer that is equivalent to their “as quenched” hardness. With NAK55, no post EDM stress relieving is ever needed.

Age hardening eliminates the stresses inherent in quenched and tempered steels. NAK55 is dimensionally stable and predictable over many years of heating and cooling cycles.

VAR processing with uniform grain structure facilitates superior nickel, chrome, titanium, and other coatings. Unique chemistry responds extremely well to ion-nitriding.

Exceptionally clean steel with uniform grain structure polishes beautifully allowing superior texturing and photo etching.

NAK55 is not quenched to achieve hardness. Therefore, it does not have the stresses inherent in quenched and tempered steels. NAK55 never needs stress relieving due to heavy machining because of uniform hardness throughout! It has excellent dimensional stability during mold construction and while in service. Even after long mold runs, cool-down, and subsequent re-heating, the material maintains dimensional stability.

NAK55 is a precipitation hardening steel. The formation of the precipitates occurs at a temperature range between 932° and 968°F. Exposure to temperatures below the original precipitation hardening temperature has no effect on the grain structure of the steel. Therefore, even molds in service at high operating temperatures maintain dimensional stability. Similarly, nitriding or Physical Vapor Deposition (PVD) treatments done below the precipitation hardening temperature range do not cause distortion. This is a tremendous advantage over typical mold steels that have been tempered at low temperatures to maintain hardness.

The precipitates, which account for the steel’s hardness, begin to grow in size if the original hardening temperature range for NAK55 (932°–950°F) is exceeded for an extended period of time. This results in a loss of hardness and toughness, and accompanying dimensional change.

Re-solution heat-treating and subsequent precipitation hardening can recover hardness and toughness, but dimensional changes will have occurred.

NAK55 is a low carbon steel that acquires hardness by precipitation hardening. Very small carbide structures and precipitates are formed during the hardening process.

Care is required when NAK55 steel slides against itself in molding situations. The use of dissimilar metals, with a 10 HRc point difference on the mating surfaces, is advisable in slide situations. Alternatively, you may change the hardness of the surface of similar metals by plating, nitriding, or applying other types of coatings.

The tolerance between mating surfaces on angle shut-offs is very important, and proper setting of the mold is required. The recommended tolerance at molding temperature is .0008″ per side for ABS, Polypropylene, Polyethylene, etc. A .0004″(.01mm) per side clearance is required for Polyacetals or nylons, due to their lesser viscosity.

NAK55 grinds easily. It is recommended that it be ground wet

NAK55 drills easily. The cutting speed should be lowered as the drill diameter increases. A smaller than standard twist angle and shorter length will reduce the danger of broken tools. Peck cycles are recommended, as NAK55 is a chip packing material.

NAK55 is a 40 HRc steel. The following is recommended to facilitate tapping:

1. Use a new, sharp, premium grade of tap, TiN coated and spiral pointed.
2. Use a tapping oil or highly chlorinated sulfurized oil. A mixture of 50% kerosene and 50% cutting oil also works well.

Copper or graphite electrodes are suitable, or the steel may be used as an electrode when burning mating halves together to achieve a fit.

The recast layer for NAK55 is soft (approximately 32 HRc). Other low alloy grades such as P20, or more highly alloyed steels such as S7 and H13, have recast layers equivalent to their high, as-quenched hardness. Because the EDM white layer must be removed, the subsequent stoning or grinding of NAK55 is much easier than with other steels.

NAK55 may develop a striped pattern with a low amperage finishing burn.

NAK55 is a Vacuum-Arc Remelted steel of exceptional cleanliness and as such, it polishes beautifully.

Normal polishing techniques will yield excellent results. All stoning operations should be done with reciprocating tools.

Note: No rotary stoning.

NAK55 steel contains uniformly dispersed manganese sulfides. These sulfides reduce friction between the steel and the stones and actually improve polishability. However, if a circular motion is used, the sulfides can be lifted up from the surface creating the appearance of pits. Therefore, it is important to use a reciprocating stoning technique and light pressure during buffing.

Due to the sulfide content of NAK55, do not exceed 3000 grit Diamond Compound.

Prior to making a welding repair, preheat the piece to be repaired by slowly heating in a furnace or with a gas burner to between 600° and 750°F. Heating from the bottom is recommended if a gas burner is used. Uniform temperatures within the recommended range should be maintained during the entire period of time required to complete the repair. Ideally, the entire mold should be heated in a furnace to achieve uniform temperatures. This is easy to do for small molds, but may not be practical, or possible, for large molds. Localized preheating is the only option for large molds, and attention must be paid to the following:

1. Preheating temperature must be achieved to at least 2″ away in all directions from area to be welded.
2. Use an oxygen-propane gas burner with a low flame temperature. Heat the mold carefully and gradually, maintaining a distance of 18″ between the flame and the mold surface.
3. Apply a temperature choke or a surface contact thermometer to accurately measure the preheating temperature.
4. Re-heat as necessary during welding to maintain the above 600°F range.

1. Use DC normal polarity
2. Use lowest possible amperage for the job
3. Use backhand welding – weld away
4. Use smallest diameter rod possible
5. Weld small beads
6. Peen weld as necessary
7. Upon completion of welding, proceed immediately to post-weld heating procedure

It is imperative that the following procedure be carefully followed to assure the welded section is completely restored to a uniform hardness:

The weld-repaired piece should be heated to between 860°F-940°F and held at this range for a minimum of one (1) hour to re-age. This re-aging process should be conducted immediately after welding. Heating with a furnace is highly recommended, however a gas burner may be used as a last resort. If a gas burner is used, heating from the bottom is recommended, however the entire welded area, and as much as 2″ surrounding the weld, must be kept in the post heat temperature range for a minimum of one (1) hour. Cool slowly to room temperature.

Note: Post-weld heating should be performed after every three layers of weld buildup in order to alleviate welding stress and avoid over-aging of adjacent parent metal.

NAK55 is a Vacuum-Arc Remelted steel of exceptional cleanliness and as such, it polishes beautifully.

Normal polishing techniques will yield excellent results. All stoning operations should be done with reciprocating tools.

Note: No rotary stoning.

NAK55 steel contains uniformly dispersed manganese sulfides. These sulfides reduce friction between the steel and the stones and actually improve polishability. However, if a circular motion is used, the sulfides can be lifted up from the surface creating the appearance of pits. Therefore, it is important to use a reciprocating stoning technique and light pressure during buffing.

Due to the sulfide content of NAK55, do not exceed 3000 grit Diamond Compound.