In many of today’s applications for powder metallurgical steels, the requirements on the material increase consistently. Tool users are seeking improvements to achieve increased cutting speeds and feed rates, greater reliability, longer life, better tolerances, and finer surface finish. These in turn call for better substrate materials to be able to optimize parameters and enhance ultimate tool performance.
In the early sixties Erasteel — the world’s leading high speed steel supplier–pioneered the development of powder metallurgical high speed steels. These are known under the registered trade name of ASP(r). Their outstanding combination of high hardness, uniformity of structure, and excellent strength has given the material the justifiable reputation of an excellent substrate material for many kinds of tools. Gear cutting tools such as hobs, shaper, and shaver cutters are among the many and varied applications of ASP.
Erasteel’s latest improvement in the manufacture of ASP is identified as the DVALINÅ process, after the Norseman who crafted the famous sword “Tirfing” used to win many glorious battles. This new development reduces the content of non-metallic inclusions from their already extremely low levels by another 90 percent, with a concomitant increase in fracture strength of 20 percent. With a stronger and cleaner material, the performance of tools and components in many situations — especially under severe operating conditions — will improve, not least for gear cutting tools.
ASP steels are produced through exposing molten steel with nitrogen gas jets, forming small spherical droplets, which then solidifies into powder. After cooling, the powder is put into capsules before being hot isostatic pressed to full density. This process ensures a homogenous distribution of alloying elements and results in a uniform distribution of small carbide particles.
Non-metallic Inclusions in PM-HSS
The critical properties of hard materials like high speed steels (HSS) are controlled by the larger imperfections in the material. Tensile strength, impact toughness, and fatigue resistance are all limited by these larger imperfections and, when in critical positions in a tool, they can cause failure. Efforts to improve the performance of hard materials necessitate the study of these imperfections through detailed evaluation of inclusion/carbide size and dispersion. With such an evaluation in hand the view is to eliminate imperfections entirely or, more realistically, reduce their frequency and, most particularly, their size. No matter whether the imperfection is present in the body of the material or on the tool surface, it is the size of any imperfection in a critical position that limits the material strength. A non-metallic inclusion present in the finished tool surface can lead to premature failure in the same way as a surface imperfection caused by poor grinding.
High-speed steels have a high content of carbon, tungsten, molybdenum, and vanadium. These elements form carbides, which are essential to give the hardness and, even more, the wear resistance expected from the steel in use. However, during the solidification of conventional high-speed steel, larger carbides and carbide segregations are also being formed, to the disadvantage of the material properties. These carbides and carbide segregations are the main limiting factors for the mechanical properties of the material, but also normally for the tools made of it. In the late sixties the limitations in strength, impact toughness, and fatigue resistance of the HSS were the main reasons for the development of the powder metallurgical (PM) process for the manufacturing of high-speed steels. The PM route brought tremendous improvements to the properties of the high-speed steels as a result of fine carbides with a homogeneous distribution. Tensile strength, impact toughness, and fatigue resistance improved due to the reduced size and amount of critical crack initiation points in the gas-atomised ASP steels.
After some years of use, it became clear that the imperfections limiting the mechanical properties of PM-HSS were not large carbides or segregations of carbides — like in the conventional HSS — but by non-metallic inclusions (NMI), typical for all types of steels. Their distribution and size can affect the mechanical performance of the steel and tools through the same mechanism as larger carbides or carbide segregations by acting as stress raising crack initiating points.
In the nineties Erasteel developed and introduced the process of electro slag heating (ESH) at its plant in S–derfors, Sweden. ESH enabled heating the steel in a large tundish.
This new process step rendered a massive 90-percent reduction in the number of NMIs present in the ASP grades.
Despite these major improvements, ongoing evaluation of ASP grades and tool performance showed that the mechanical properties were likely to be improved if the level of NMI could be reduced even further.
In a project that Erasteel started in 2002, the non-metallic inclusions were studied in detail: how and where they were being formed, and where they grew in the process chain. The sources to the NMI have now been eliminated/minimised through innovative technical improvements, and the whole production chain follows best possible practice, based on the material properties achieved. The new process to produce ASP steels has been given the name DVALIN.
Compared to the earlier generation of steels, depending only upon ESH, another 90 percent of the non-metallic inclusions have been removed with DVALIN. This has added 20 percent to the fracture strength of the ASP steels, contributing significantly to the performance for tools in terms of reliability, feed rate, tool life, and high finish surfaces, etc. (See Figure 1 and Figure 2.)
Note: When using the latter generation of tool materials with refined microstructure, the surface finish becomes of great importance to maximise the performance of various tools. Improvement of tool surfaces is definitely one potential area of improvement, especially in combination with for instance PM-HSS.
Benefits with DVALIN
For high performance cutting tools such as hobs, broaches, end mills, and taps, it can be the homogeneity and cleanness of the steel that make the difference between success and failure of a tool.
In other types of applications, such as cold work tools or components, the very fine carbides and low level of NMI distributed throughout the homogeneous structure of ASP may become highly beneficial. Along with good grinding and polishing techniques by our toolmaker customers, the material contributes to the provision of tooling with a near mirror-like finish and extended working life.
In many applications, tools or components operating under conditions of oscillating loads make severe demands on the fatigue resistance of the PM-HSS. DVALIN answers all these questions owing to its enhanced fatigue strength consequent of the low level of NMI and fine carbides distributed optimally throughout the ASP grades. (Figures 3, 4, 5, 6)
What are Non-Metallic Inclusions?
All steels contain more or less non-metallic inclusions (NMI) deriving from scrap, ferro-alloys, or the general melting environment. Although it is accepted that, as NMIs are less dense than the molten steel, many will simply float to the surface during melting and be discarded as part of the slag, but a number can remain in the molten steel. In order to stimulate the removal of these remaining NMIs, processes such as argon bubbling or electromagnetic stirring can be introduced during melting. Another alternative is to carry the entire melting process through under vacuum. Whatever technique is applied, the manufacture of “clean steel” demands tight control of the melting process.
With reference to the American standard ASTM E45, non-metallic inclusions can be divided into: A) Ductile; B) Brittle; C) Brittle Ductile, and; D) Un-deformed. Each group is then further subdivided on seven levels based on the number of inclusions. The A-inclusions are often sulphides like MnS and they deform with the steel during forging, rolling, and other forming operations. B-, C-, and D- inclusions are oxides of different analysis. Aluminium, silicon-calcium, and titanium are some of the element found in these groups. B-inclusions are brittle and crack up to a rather harmless configuration during the metal forming process. C- and D-inclusions are more severe since they deform very little or not at all, and thus stay as large crack initiation points.
By consistent investigation and ongoing process development, the Erasteel PM-HSS grades, ASP, have reached unprecedented levels of cleanness. The DVALIN process allows the production of the purest PM-HSS powders available globally and assists our customers in manufacturing long lasting, reliable, and maximum productivity tooling.
* ASP(r) is a registered trademark of Erasteel