Overheated and Burnt Steels: Causes and Reclamation | Metallurgy

In this article we will discuss about:- 1. Causes of Overheated and Burnt Steels 2. Detection of Overheated and Burnt Steels 3. Reclamation.

Causes of Overheated and Burnt Steels:

When steels are heated above the upper critical temperatures, austenite grains coarsen. If the coarsening is not excessive, normalising, or annealing can refine the grains to normal size. If the heating of the steel is continued to be close to solidus, then two defects are induced in the steels—first, the ‘overheating’ which impairs the properties of the steels, and if the normal properties cannot be restored by further heat treating and/or mechanical working, then the steel has the second defect called burning.

Slight reduction in ductility during the subsequent forging indicates overheating, whereas if the forging break up under the hammer, then the steel is burnt.

Although the ductility is reduced by overheating, but the room temperature fracture is ductile, resulting in only small reductions in fracture toughness. Overheating is due to the solution of MnS in austenite at high temperatures, and its subsequent precipitation during cooling as fine particles (0.5 – 1.0 µm) of MnS at austenite grain boundaries (Fig. 2.37 a). That is why the room temperature fracture follows the path of former austenite grain boundaries.

The extent of overheating depends on the extent of MnS precipitation. Impact and tensile properties are not seriously impaired in the initial stages of overheating, and the parts may pass into service without being detected, but parts like connecting rods, or gears may fail prematurely by fatigue.

Overheating can be eliminated in steel, if Mn content is increased in the steel, which in turn reduces the solubility of sulphur in solid solution in austenite at a given temperature, or by adding Ca, or Ce to take care of the sulphur in steel. Good amount of grain-boundary-sulphide-precipitates cannot be effaced by any practical heat treatment, but can be by mechanical working at high temperatures.

In ‘burning’, formation of a film of sulphur-rich liquid takes place at the austenite grain boundaries. The damage is total and permanent, as the steels than lose their ductility and disintegrate during deformation. Burning too can be eliminated by treating the steels with Ca, Ce, or mixed rare-earth additions to form less soluble sulphides.

Overheating is supposed to occur only when the steel is heated above a certain minimum temperature below solidus, where dissolution of MnS takes place in austenite (and subsequently precipitated), and burns when there is incipient fusion. The transition between the severely overheated and the burnt steel is not clearly defined, as it is difficult to judge the temperature of the start of incipient fusion, and thus, two terms are used rather loosely.

The temperature at which these phenomena occur are not likely to be reached during heat treatment procedures, but may be attained during heating for forging (or in an attempt to increase the production by forging in one heat, when it requires two), or in the heat-affected-zones adjacent to fusion welds. Industrially overheating and burning seriously restrict the upper forging temperature.

Detection of Overheated and Burnt Steels:

The nicked fracture test performed preferably on hardened and tempered steel, in the initial stage of overheating, shows evidence of granularity in the normal fibrous fracture and becomes completely granular on pronounced overheating.

Burnt steels show evidence of partial fusion on the exposed facets of granular fracture. As the granularity or faceted fracture in overheating can be suppressed by later thermal history of the steel, its absence in fracture is not clear indication of the steel not overheated. But overheating and burning can be detected metallographically.

One of the oldest and widely used techniques is to use a 10% aqueous solution of nitric acid and sulphuric acids. The polished specimen is immersed in the mixture for 30 seconds, removed, washed and the black surface film swabbed away.

This process is repeated three times after which the specimen is lightly polished. The etchant attacks the prior austenite grain boundaries. Under the microscope, overheated steel gives a network of black grain boundaries with a lightly attacked back ground; the boundaries appear white in burnt steels.

Fig. 2.37 (b) illustrates overheating by etchant electrolytic ammonium nitrate at 100 X magnification of fracture surface section, whereas 2.37 (c) illustrates burning by etchant acids at 50 X magnification.

Reclamation of Overheated and Burnt Steels:

Severely overheated and burnt steels cannot be reclaimed.

The less severely overheated steels can be recovered by one of the following heat treatment cycles:

(a) Repeated normalising (5 or 6 times) starting at temperatures 50-100 higher than normal temperature and finally, normalising at the normal temperature. The size of the new austenite grains is smaller than old austenite grains but new austenite grains size is not recovered in a single reaustenitising treatment of a very coarse grained structure. Thus, several cycles are needed.

(b) Repeated oil hardening and tempering after long soaking at 950 to 1150°C in carburising atmosphere.

(c) Hot working at high temperature like forging can relocate the sulphides from grain boundaries.