_Heat Treatment L_

After forging or machining: 1. Carbonize between 1,600F. and 1,750F. (1,650-1,700F. desired).

2. Cool slowly in the carbonizing mixture.

3. Reheat to 1,400-1,500F.

4. Quench.

5. Reheat to 1,300-1,400F.

6. Quench.

7. Reheat to 250-500F. and cool slowly.

_Heat Treatment M_

After forging or machining: 1. Heat to 1,450-1,500F.

2. Quench.

3. Reheat to 500-1.250F. and cool slowly.

_Heat Treatment P_

After forging or machining: 1. Heat to 1,450-1,500F.

2. Quench.

3. Reheat to 1,375-1,450F. slowly.

4. Quench.

5. Reheat to 500-1,250F. and cool slowly.

_Heat Treatment Q_

After forging: 1. Heat to 1,475-1,525F. (Hold at this temperature one-half hour, to insure thorough heating.) 2. Cool slowly.

3. Machine.

4. Reheat to 1,375-1,425F.

5. Quench.

6. Reheat to 250-550F. and cool slowly.

_Heat Treatment R_

After forging: 1. Heat to 1,500-1,550F.

2. Quench in oil.

3. Reheat to 1,200-1,300F. (Hold at this temperature three hours.) 4. Cool slowly.

5. Machine.

6. Reheat to 1,350-1,450F.

7. Quench in oil.

8. Reheat to 250-500F. and cool slowly.

_Heat Treatment S_

After forging or machining: 1. Carbonize at a temperature between 1,600 and 1,750F.

(1,650-1,700F. Desired.) 2. Cool slowly in the carbonizing mixture.

3. Reheat to 1,650-1,750F.

4. Quench.

5. Reheat to 1,475-1,550F.

6. Quench.

7. Reheat to 250-550F. and cool slowly.

_Heat Treatment T_

After forging or machining: 1. Heat to 1,650-1,750F.

2. Quench.

3. Reheat to 500-1,300F. and cool slowly.

_Heat Treatment U_

After forging: 1. Heat to 1,525-1,600F. (Hold for about one-half hour.) 2. Cool slowly.

3. Machine.

4. Reheat to 1,650-1,700F.

5. Quench.

6. Reheat to 350-550F. and cool slowly.

_Heat Treatment V_

After forging or machining: 1. Heat to 1,650-1,750F.

2. Quench.

3. Reheat to 400-1,200F. and cool slowly.

RESTORING OVERHEATED STEEL

The effect of heat treatment on overheated steel is shown graphically in Fig. 65 to the series of ill.u.s.trations on pages 137 to 144. This was prepared by Thos. Firth & Sons, Ltd., Sheffield, England.

[Ill.u.s.tration: FIG. 65.--Chart of changes due to heating and cooling.]

The center piece Fig. 65 represents a block of steel weighing about 25 lb. The central hole accommodated a thermo-couple which was attached to an autographic recorder. The curve is a copy of the temperature record during heating and cooling. Into the holes in the side of the block small pegs of overheated mild steel were inserted. One peg was withdrawn and quenched at each of the temperatures indicated by the numbered arrows, and after suitable preparation these pegs were photographed in order to show the changes in structure taking place during heating and cooling operations. The ill.u.s.trations here reproduced are selected from those photographs with the object of presenting pictorially the changes involved in the refining of overheated steel or steel castings. Figures 66 to 79 with their captions show much that is of value to steel users.

[Ill.u.s.tration: FIG. 66.--The structure of overheated mild steel from which all the pegs were made (magnified 25 diameters). The pegs withdrawn at 720C., or earlier, had this structure and were quite soft.]

[Ill.u.s.tration: FIG. 67.--Peg withdrawn at 750C. (magnified 25 diameters). The structure is apparently unaltered, but the peg was hard and, unlike the earlier ones, would not bend double.]

[Ill.u.s.tration: FIG. 68.--A portion of 66 magnified 200 diameters to show that the dark (pearlite) areas are laminated.]

[Ill.u.s.tration: FIG. 69.--A portion of 67 magnified 200 diameters, showing that pearlite areas are no longer laminated and providing reason for observed hardness.]

[Ill.u.s.tration FIG. 70.--Peg withdrawn at 780C. (magnified 25 diameters), showing inter-diffusion of transformed pearlite and ferrite areas.]

[Ill.u.s.tration: FIG. 71.--Peg withdrawn at 800C. (magnified 25 diameters), showing inter-diffusion so far advanced that the original outline of the crystals is now only faintly suggested.]

[Ill.u.s.tration: FIG. 72.--Peg withdrawn at 850C. (magnified 100 diameters) after inter-diffusion was completed. Note the regular outlines and the small size of the crystals as compared with 67.]

[Ill.u.s.tration: FIG. 73.--To facilitate comparison 67 was enlarged to the same magnification as 62, and the one superimposed on the other. The single large crystal occupied as much s.p.a.ce as 8,000 of the smaller ones.]

[Ill.u.s.tration: FIG. 74.--The peg withdrawn on cooling at 800C.

(magnified 100 diameters) shows the first reappearance of free ferrite. All pegs withdrawn at higher temperatures were like Fig.

72.]