Annealing of casting

In the case of steel castings full annealing is the only way for completely effacing the coarse grains and Widmanstätten structure, with its associated brittleness.

The following properties of a 0,3 % carbon steel illustrate the improvement obtained by annealing:

Table 1

	BH	YP MPa	TS MPa	El %	Cold Bend	Izod J
Cast	160	309	448	6	45°	12
Annealed 880°C, 6hr, furnace cooled	153	247	479	24	165°	32

The Widmanstätten structure can be modified into a "feathery" arrangement of the ferrite by the influence of silicon. This is shown in Fig. 1 which consists of a portion of the boundary-ferrite, Widmanstätten and feathery structures. Fig. 2 shows the macroform of the primary crystals, revealed by the segregation of the impurities. The effects of this segregation have to be effaced as much as possible by annealing and this necessitates temperatures higher than those used for worked steels.

Figure 1. Micro-structure of same steel sho-wing part of ferrite network, Widmanstätten and feathery structure. Ferrite-white. Pearlite-dark ( x 80)

Figure 2. Macro-structure of cast steel revea-ling large prirmary austenite crystals due to presence of impurities (x4)

An imperfect anneal is illustrated by Fig. 3, in which the original cast structure is still outlined by the deposition of the ferrite in the old positions, especially around the inclusions. They can be prevented by double annealing.

Double annealing consists of heating the steel to a temperature considerably over the A3 point, cooling rapidly to a temperature below the lower critical range and then immediately reheating to a point just over the upper critical point (Ac3), followed by slow cooling.

This method is particularly useful for castings. The high temperature treatment effaces the strains, coalesces the sulphide films in the ferrite which embrittle the steel and produces homogenity by rapid diffusion. The quick cooling prevents the coarse deposition of the ferrite in the large grains, but tends to harden the metal. The second heating refines the coarse grains and leaves the steel in a softened condition. A typical structure, which is shown in Fig. 4, should be compared with Fig. 3.

Figure 3. Same steel imperfectly annealed ferrite formed in masses outlining original cast structure (x80)

Figure 4. Same steel properly annealed: ferrite and perlite uniform and fine (x80)

Softening Tool and Air-hardening Steels

To soften high carbon and air-hardening steels, in order to allow machining operation to be carried out, they are heated just below the lower change point (650-700°C), causing. the cementite to balls-up into rounded masses (i.e. spheroidising anneal). When the cementite is in this condition high carbon steels can be cold drawn; but too high a temperature causes pearlite to be reformed, with consequent high resistance to deformation.

It should be remembered, however, that coarse laminated cementite spheroidises extremely slowly, and the above treatment is therefore carried out on a "hardened" material, obtained by suitable cooling from above A3 or after cold working. A short cycle anneal consists of heating just above Ac1, cooling below Ar1 and then raising temperature just below Ac1 for 8 hours. Although the softest condition is obtained when the large globules of cementite are embedded in the ferrite, a smooth machined surface is difficult to obtain due to tearing. Groups of large globules cause failure of sharp-edged cutting tools.

During the subsequent hardening operations, the time required to dissolve fine spheroidised cementite is less than for the lamellar type. This property is being used in hardening thin sections, such as safety razor blades and needles, in order to reduce decarburisation.

Annealing and Hardening Temperatures for Tool Steels

The annealing or hardening temperatures of steels containing more than 0,9% carbon is just above the lower critical point (730-790°C) instead of above the upper range.

Fig. 5 shows the appearance of a 1,3% carbon steel cast, in which the cementite exists as brittle networks and plates. This type of structure must be replaced by rounded particles of cementite in a fine pearlite before hardening, otherwise cracks will propagate through the continuous masses of brittle cementite.

Figure 5. As cast: cementite network and plates in pearlite (x 100)

Figure 6. Heated to 1050°C and quenched in water. Large grains (x 100)

Figure 7. Cementite globules in properly hot-worked steel (x 200)

Figure 8. Cementite globules in martensite, in hardened steel (x 200)

The upper critical line rises steeply with increasing carbon content above 0,87% and an excessively high temperature is required to dissolve all the free cementite. This tends to develop coarse austenite crystals which cause the steel to become brittle and cracks to form on quenching. This grain growth is shown on Fig. 6, which is the structure of steel quenched from 1050°C. On the other hand, particles of cementite restrain grain growth.

Forging is, therefore, carried out through the critical range in order to disperse the free cementite. This is followed by annealing at about 760°C (just above Ac1), to ball-up the free cementite and to remove strains. Fig. 7 shows the structure formed. Even a fine cementite network structure would cause trouble when drastic quenching is used, such as for files. Fig. 8 shows the structure of a steel hardened from 760°C, consisting of particles of cementite dispersed in a matrix of martensite.

Products of Quenching: Cnstituents of Hardened Steel

The equilibrium diagram indicate the changes which occur under the slow cooling conditions of annealing. The rapid rates of cooling necessary to harden a steel cause the austenite to persist to a lower temperature and to transform into a variety of micro-constituents discussed below.

Austenite is sometimes present with martensite, in drastically quenched steels (Fig. 9). It cannot be completely retained in carbon steels by even drastic quenching, but suitable additions of alloying elements allow the retention of this constituent, for example 18/8 austenitic stainless steel. This austenite consists of polyhedral grains, showing twins (Figs. 9 and 10). It is non-magnetic and soft.

Martensite is the hardest constituent obtained in a given steel, but the hardness increases with the carbon content of the steel up to 0,7%. The micro-structure exhibits a fine needle-like structure, which becomes more pronounced when the steel is quenched from high temperatures. See Figs. 9 and 11.

Figure 9. Forms of carbide in micro-constituents in steel

Figure 10. Martensite needles (dark) in austenite (x1200)

Figure 11. Steel (C, 0,4) quenched from between A1 and A3. Undissolved ferrite arou-nd inclusion in martensite (x 100)

Figure 12. Martensite and quench crack. Steel (C, 0,5) quenched in water from 900°C(x400)

Figure 13. Nodular troostite in martensite (x400)

Figure 14. Sorbite in quenched and tempered (600°C) steel (C, 0,5) (x500)

Figure 15. Case-hardened screw. Cracked martensitic case (white), martensite and fer-rite core (x30)

The nature of martensite has not been definitely agreed upon, but for the present purpose it might be considered to be highly stressed a-iron which is supersaturated with carbon. The fc.c. lattice of g-iron is equivalent to a body centred tetragonal lattice with ratio 1,414. The tetragonal lattice of martensite is formed from this by compressing its height and increasing its cross-section. A slight further compression to give a ratio of 1 gives rise to a-iron.

Bainite, which occurs in alloy steels, has a rapidly etched needle-like structure, somewhat resembling tempered martensite. As the temperature of its formation becomes higher the acicular nature becomes less accentuated, the needles increasing in size

Troostite (nodular) rapidly etches black and is practically unresolvable under the ordinary microscope. Special microscopic technique has shown that nodular troostite is a mixture of radial lamellae of ferrite and cementite. Therefore it differs from pearlite only in degree of fineness and carbon content which is the same as that in the austenite from which it is formed. Figs. 9 and 13 show a typical martensite-troostite structure; nodules outline the boundaries of the original austenite grains. Troostite is softer than martensite and small amounts in the steel lessen the risks of cracking and distortion.

Some confusion arises as to the nomenclature of micro-constituents found in hardened and tempered steels. The terms troostite and sorbite are frequently used to indicate constituents formed during quenching and also during tempering. In the former case (quenching) the cementite always occurs in a laminated form, while in the latter (tempering) it has a granular form. Hence, the term troostite has been adopted in this book for constituents possessing a laminated structure. Sorbite is used to denote the granular structures.

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