In the modern steel industry, steel quality does not depend only…
Special steel production processes: the importance of milling
In special steel production, surface quality and material integrity are crucial aspects to ensure that the final components meet the most stringent requirements. During hot plastic deformation stages (forging, rolling) and heat treatments, surface defects can occur that compromise the steel’s integrity. For this reason, an additional machining process, such as milling, is essential to remove imperfections and ensure steel that is ready for industrial applications. In this article, we will explore the importance of milling and why it is so relevant in the special steel production cycle.
Why milled material
During the production of steel blocks, following forging processes and subsequent heat treatments for refining and homogenizing the structure, several phenomena occur, including:
- Surface oxidations and decarburizations
- Laps, folds, cracks
- Deformations
After solidification, the steel blocks remain at temperatures above 500°C to allow for subsequent forging. In this state, the steel reacts with the oxygen present in the atmosphere, generating layers of decarburization and surface oxidation. These reactions alter the surface characteristics of the material, making further machining operations necessary to obtain a product free from surface defects.
Phenomena during forging
During the forging stages, the steel, subjected to plastic deformation, undergoes significant compression from hammers and presses. This process can generate surface defects, such as laps and cracks. Although steel mills have the expertise to minimize them, these phenomena are still inevitable and natural.
Deformations and subsequent processing
In addition to defects related to hot plastic deformation, other factors to consider are block deformations caused by production processes and surface conditions related to oxy-cutting operations. Once the desired shape is achieved, it is therefore essential to perform subsequent material removal through milling in order to use the block in the construction of mechanical components free from surface defects that could compromise their performance in service or during subsequent steel processing stages.
Regulations and standards for quality.
To ensure steel free from defects caused by the previously described production processes, specific standards are applied to assist users, such as DIN 17350: 1980: TOOL STEELS – TECHNICAL CONDITIONS OF DELIVERY. This standard provides technical specifications for the delivery of tool steels (cold, hot, and high-speed) concerning the dimensions and tolerances of rough, milled, and finished blocks, with particular usefulness regarding the minimum material to be removed from the rough block.
The advantages and importance of milling.
Milling allows for much more precise surface control compared to raw material, also enabling more accurate and reliable UT (ultrasonic) inspections. This highlights the importance of milling in ensuring that steel blocks are free of defects and ready for machining.
By using specific formulas, it is possible to determine the minimum milled dimensions to which the raw steel/forging block must be brought in order to guarantee that it is free from the aforementioned natural defects.
Df: Milled thickness
Dr: Raw thickness
Bf: Milled width
Br: Raw width
Thanks to the milled surface quality of the material, it is also possible to perform UT inspections that are much more precise, accurate, and reliable compared to the same material with a rough surface; thus ensuring higher quality standards.
It is at the user’s risk to remove an amount of excess material from the forging rough stock that is less than what is specified by the relevant standards.
Concrete examples of chip removal.
It is easy to notice weight differences of about 15% between the supply of milled materials and the corresponding raw blocks supplied. Moreover, this does not take into account the related costs and time of chip removal.
The importance of milling: conclusion.
The removal of material by milling is essential to ensure the integrity and quality of special steel intended for critical mechanical components.
The importance of milling lies in its ability to eliminate surface defects and prepare the material for the most demanding applications. Thanks to the use of specific standards and advanced tools, it is possible to ensure that the final product is free of imperfections and compliant with the strictest industrial standards.
