Protective Coatings Increase Material Yield and Reduce Costs
This article introduces a practical technique that enables the reduction of burning loss or mill scale and increases yield in most kinds of steels. The technique is most successfully adopted in open-die hot forging and hot rolling of stainless and other expensive alloy steels.
In open-die forging, hot rolling and even closed-die forging of stainless and alloy steels, the major factor accounting for reduced yield is burning loss, also known as scale loss or mill scale caused by surface oxidation. Oxidation and decarburization of steel take place when billets or ingots are heated in a reheating furnace in the presence of air or products of combustion. Apart from burning loss of between 1.5-4.0%, oxidation leads to many other problems such as its removal, scale pit marks, poor surface finish, rejections and increased use of expensive operations like grinding, acid pickling, etc.
This article introduces a practical technique by which material losses due to oxidation can be minimized.
Understanding Burning Loss Due to Oxidation
For the purpose of hot rolling, when billets or ingots are heated in an open furnace in the presence of air or products of combustion, the surface phenomenon of oxidation takes place. Oxidation causes the immediate corrosion of steel at high temperature and creates a layer of metal oxide, or mill scale, on the surface of ingots and billets. Decarburization is a simultaneous reaction that takes place along with oxidation, though decarburization occurs only in certain grades of steel.
The oxidation of steel is caused by the presence of oxygen, carbon dioxide and/or water vapor. The general reactions are:
O2 + 2 Fe ↔ 2 FeO
O2 + 4 FeO ↔ 2 Fe2O3
CO2 + Fe ↔ CO + FeO
CO2 + 3 FeO ↔ Fe3O4 + CO
Oxidation of steel may range from a tight, adherent straw-colored film that forms at a temperature of about 180°C (356°F) to a loose, blue-black oxide scale that forms at temperatures above about 450°C (842°F) with resultant loss of metal or reduced yield.
Harmful Effects of Oxidation
Oxidation leads to dimensional inaccuracies and material loss, insofar as extra material allowance needs to be made for scaling. Surface quality is often negatively affected because of scale-pitting, which is especially true in the case of nickel-bearing grades of steel. Throughout the industry this burning loss (scale loss) is simply treated as waste, and not many practical solutions are available to stop or reduce it. Problems like thick adherent scaling, scale pit marks or decarburization are dealt with mainly by grinding off the scale or decarburized layer after the hot-forging or hot-rolling process.
Preventing or Reducing Burning Loss Due to Oxidation
The prevention or substantial reduction of oxidation is better than a cure for the problem of material loss due to scaling because it is also profitable. However, most available solutions pose a number of practical difficulties. Capital-intensive special furnaces and the availability of human resources for using these high-end furnaces is a major issue. Many small hot-rolling organizations cannot afford these solutions, yet they are under mounting pressure to increase yield and reduce costs.
To this end, the use of a protective anti-scale coating has proven to be a logical solution to the problem of scaling and decarburization in open-die hot forging and hot rolling.
Characteristics of Protective Coating and its Use
The use of a protective coating (Figure 1) has been found beneficial and cost-effective in the prevention or reduction of the deleterious effects of scaling. An anti-scale coating is applied on billets/ingots to be heated before charging them into the furnace. This protective coating acts as a barrier between the metal and oxygen. Before heating, care is taken to apply a uniform, impervious coating on the billet to be heated. This ensures a substantial reduction in oxidation and, thereby, a reduction in burning loss or mill scale. Heat transfer from the heating medium to the metal is unaffected by the coating, which also reduces decarburization on billets during hot-rolling operations.
Finally, the coating has no reaction with the steel surface; no release of toxic fumes during use, hot forging or storage; and it is otherwise nonhazardous.
Protective Coating Increases Yield
Because of the reduction in oxidation caused by the anti-scale coating during hot forging and hot rolling, the scale thickness is considerably reduced. Figures 2 and 3 are of mill scale that was generated on billets during reheating for hot rolling with and without using protective anti-scale coating.
For this example the following conclusions may be drawn (Figure 4):
• Total reduction in scale = 0.95 mm
• Percentage reduction in material loss due to mill scale by using anti-scale coating = 56.98%
• On average, approximately a 70% reduction in mill scale is achieved by the use of anti-scale coating in open-die hot forging and hot rolling of stainless steel. This increases material yield and reduces cost.
In another trial, a number of mild-steel, hot-rolled billets without coating were compared with the hot rolling of similar billets with a single coating and double coating of anti-scale protection. The results are provided in Table 1.
For this example, the following observations were made:
• Scale loss of 2.14 kg was observed on billets without coating.
• Scale loss of 1.140 kg was observed on billets with a double layer of protective anti-scale coating.
• Percentage reduction in mill scale loss due to use of protective anti-scale coating = 47.00%
Other Benefits of Coating Use in Stainless Hot Rolling and Open-Die Forging
There are several additional benefits available when using coatings in stainless steel hot rolling and open-die forging.
Reduction in Grinding, Acid Pickling Time of Hot-Rolled Products
Operations like grinding, acid pickling, etc. do not add value, but they are expensive and time-consuming procedures that are necessary to remove adherent scaling from the hot-rolled billets. The time required for operations like shot blasting, grinding, acid pickling, etc. can be substantially reduced if protective anti-scale coating is applied on components before heat treatment. The aesthetic appeal of components is automatically enhanced without much effort because scaling is either prevented or reduced by the use of an anti-oxidation coating.
Reduced Decarburization During Hot Rolling and Hot Forging
Certain grades like spring, ball bearing and rail steels are susceptible to decarburization. Decarburization needs to be maintained to the required specification during hot-rolling of these special grades. Due to unforeseen mill conditions such as unplanned downtime, however, billets may remain in the furnace for a longer time than expected. Also, when the plant is closed (on weekends, for example) the furnace is shut off abruptly, subjecting the billets to prolonged heating inside the furnace. If billets are not coated, it becomes difficult to guarantee their level of decarburization. Use of protective coatings on billets before reheating enables better decarburization level control. This enables the hot-rolling mill to guarantee their customers that decarburization will always be less than the upper control limit. Reduced decarburization on vehicle leaf springs leads to increased fatigue strength of the leaf springs and greater reliability. Leaf springs heated after applying protective coating show substantially reduced decarburization and scaling.
Improving Surface Finish of Hot-Rolled Components and Preventing Welding of Billets During Heating
During the hot rolling of special grades of steel – like nickel (Ni) and 416 stainless steel with high sulfur content – mill scale is greatly increased. Consequently, the surface finish of hot-rolled product may be compromised due to scale pits and rolled-in scale. Occasionally, due to heavy adherent scale, the problem of roll skid is also encountered. Subsequent grinding operations are increased due to excessive mill scale. Coating the billets before charging them into the reheating furnace ensures reduced oxidation and mill scale. Additionally, coated billets yield a good surface finish of hot-rolled products free from scale pits and rolled-in scale. Sometimes billets get welded together during reheating for hot rolling and during induction heating for hot forging. Separating these billets is unproductive and preventable with the use of the protective coating.
Improving the Surface Finish of Seamless Pipes
In the manufacture of seamless pipes, machined hollow ingots are used in many places, especially in Poland. It becomes a necessity to protect the surface by protective coating when the ingot is reheated. This ensures better surface finish of the seamless pipe. This is achieved by the use of anti-scale protective coatings.
The use of a protective, anti-scale coating for steel billets achieves a 70% average reduction in burning loss (mill scale loss) and increased material yield in stainless steel open-die forging and hot rolling. Protecting billets by applying the coating ensures a number of additional benefits such as the elimination or reduction of grinding, acid pickling and other operations; reduced decarburization; prevention of billet welding in the furnace; and improved surface finish.