Steels flame hardening

PISTON RINGS are normally cast-iron of the single-piece snap-ring type, although other materials may be used when conditions require. PISTON ROD is carbon-steel, flame-hardened over packing travel area. 

PISTON ROD is carbon-steel, flame-hardened over packing travel area unless otherwise specified. 

Laser/EB harden Carbonitride/caiburize/cyanide Induction-hardened steels Flame-hardened steels... 

Fig. 3. Flame hardening (a) tempeiatuie—time heating curves of a 25 x 50 x 100 mm specimen at a feed of 75 mm /min and burner distance of 8 mm showing temperatures of A, surface B, 2 mm below surface and C, 10 mm below surface (b) hardness—depth curves for a 0.50% C steel 25 X 75 X 100 mm specimen at a feed of 50 mm /min, temperatures ia °C measured 10 mm below the surface, and burner distances ia mm, respectively, of A, 530 and 50 B, 540 and 12 C, 545 and 10 D, 550 and 8 and E, 565 and 6. Flame heating followed by water spray quenching. HV = Vickers hardness.

The dies themselves are generally constructed of medium carbon alloy steels, with flame-hardened lips and chrome or nickel plating on the flow surfaces. Insulation of the die body Is common to prevent temperature variations that could result from air drafts. Dies can be electrically heated, or steam or oil heat can be used. Cooling is usually provided by natural convection, though forced air can also be used. 

Figure 4.90 shows an example of a plastic compression mold made of the prehardened plastic mold steel 1.2738 (40CrMnNiMo8-6-4). Cracking occurred during flame hardening of the parting lines due to the absence of pre-heating. 

The flame-hardening process is used for a wide variety of applications. These include (1) parts that are so large that conventional furnace treatments are impractical or uneconomical, (2) prevention of detrimental treatment of the entire component when only small segments of die part require heat treatment, and (3) use of less costly material to obtain the desired surface properties where alloyed steels would be normally applied. Flame hardening is limited to hardenable steels (wrought or cast) and cast iron. Typical hardnesses obtained for the flame-hardened grades depend on the quench media (Table 1). The practical level of minimum surface hardness attainable with water quenching for various carbon contents is shown in Fig. 1. 

Table 1 Response of steels and cast irons to flame hardening...

To harden the steel, use tongs to hold two of the bent hairpins in the flame until they are glowing red all over. Quickly plunge the hot metals into a 250-mL beaker containing approximately 200 mL of cold water. Quick-cooling causes the crystal size to be small. 

For steel when correctly hardened (typically at 800C) and tempered at 200C, a hardness of Rockwell C52-58 would be expected. Where addidonal skin hardness is needed (as on sliding surfaces of the tool) it is possible to harden by flame or vacuum treatment, but any subsequent grinding (which may be carried on final assembly of the tool) will easily remove the surface and expose the softer substrate, which will be very prone to wear. 

Flame or induction hardened. (2) Trademark of La Salle Co. (3) Trademark of Joseph T. Ryerson Son Inc. (4) Nitrided. (5) Usually improved by hardsurfacing. (6) Trademark of Armco Steel Corp. (7) Trademark of Crucible Specialty Metals. (8) Trademark of Huntington Alloys Inc. (9) Trademark of Cabot Corp. 

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