The impact of alignment on your steel turning process

A common misconception in the metal working industry is that machining steel is simple. Experienced machinists know that turning ISO P steel is anything but. First among many concerns is the diversity of materials with ISO P classification, which range from ductile low-carbon to high-alloy steels.

Secondly, the hardness of different steels ranges significantly from one end of the spectrum to the other. The type of application varies and so do machining conditions in workshops.

Evidently, steel turning is challenging and given all the variables, the task of selecting a grade to cater to the wide range of properties exhibited by ISO P steels is even more daunting.

Grade of all trades

For any such grade, fracture resistance is paramount. As is a cutting edge capable of delivering the hardness needed to resist plastic deformation induced by the extreme temperatures present in the cutting zones.
Moreover, the grade must be equipped with a coating that can prevent flank wear, crater wear and edge build-up. Importantly, the coating must also adhere to the substrate; if it does not stick, the substrate is exposed, leading to rapid failure.

Given the array of demands, it is crucial to understand the structure of a steel turning grade in order to make an informed decision when choosing one for your application.

Structure of a carbide insert

All carbide grades contain a cemented carbide core known as a substrate. The substrate determines the toughness and strength of the grade. It also governs its resistance to plastic deformation.
The cemented carbide substrate is usually covered with a few layers of coating such as titanium carbonitride (TiCN), alumina (Al2O3) or titanium nitride (TiN), which affect the insert's edge toughness, adhesion and wear resistance properties. The recipe for superior resistance to different kinds of wear (flank, crater and edge build-up), adhesion to substrate and improved tool life is found in the microscopic details inherent in the design of the coating.

Roman shield wall

In conventional alumina coating, crystal growth direction is random, as shown in Figure 1a. If the growth in the coating layer can be controlled to ensure all the crystals line up in the same direction, as shown in yellow in Figure 1b, the result is superior wear resistance.

To help you understand the power of crystal alignment, let's consider an example from Roman history. When the ancient Roman legions were involved in a siege, they frequently deployed a shield wall known as the Testudo formation. In this formation, all the shields were aligned and tightly packed to avoid leaving any vulnerable gaps. The shield wall helped the Romans resist oncoming aggression while advancing.

The alignment of crystals in a coating layer works in a similar way: the closely packed unidirectional crystals act as a shield and provide better resistance against difficult conditions at the cutting zone.

Unidirectional crystals

Sandvik Coromant's R&D experts have found a way to control the crystal growth in the alumina coating to ensure that all the crystals line up in the same direction, with the strongest part at the top. This patented technology known as Inveio® coating is a technical breakthrough that offers inserts a new level of wear resistance and longer tool life.

The tightly packed unidirectional crystals create a strong barrier against the cutting zone and chip. This enables Inveio®-equipped grades to greatly improve resistance to crater and flank wear. Another advantage is that heat is led more rapidly away from the cutting zone, helping the cutting edge retain its shape for longer times in cut. Overall, what you get is a predictable tool with a long tool life.

With the introduction of second generation Inveio® technology as featured in Sandvik Coromant's latest steel turning grades, GC4415 and GC4425, the benefits of unidirectional coating have been further enhanced. The improved crystal orientation results in even more consistent performance and significantly improved wear resistance.

Intermittent cutting operations

Now that we've discussed the first two considerations for selecting an insert grade, namely the substrate and the coating, let's briefly look at the third: performance during intermittent cutting operations. This is an important factor as it helps to avoid sudden insert breakages.
Look for inserts that have undergone post-treatment, a process in which the insert coating is bombarded with very fine, sharp ceramic particles. Imagine a hammer striking the coating to reinforce and strengthen it. Inserts that have undergone effective post-treatment perform well during intermittent cutting.

New GC4415 and GC4425 grades

Sandvik Coromant recently launched two new ISO P steel turning insert grades. The GC4415 and GC4425 grades are ideal for manufacturers operating in mass and batch production set-ups.

Featuring a new substrate reinforced by Inveio® technology, the two grades offer reliable performance and superior wear resistance. The new post-treatment on these grades is found to boost performance in intermittent cutting operations, avoiding sudden breakages and enabling both insert grades to offer superior performance across a broad application range.
These grades have enabled customers to implement higher cutting speeds (Vc) and multiplied feed rates (Fn). In one case, a general engineering manufacturer subjected a 4140 pre-heat treated steel workpiece to multi-directional external roughing with the GC4425 insert. Compared to a competitor's ISO insert for the same process, the customer was able to achieve a 100 % productivity increase with a 50 % reduced cycle time and a 30 % cost reduction.

Machining ISO P steel is tricky. Keeping a few considerations in mind when selecting the grade, such as substrate toughness and new technological advancements in the area of material science and tooling technology, can make a huge difference to your steel turning efficiency and your machine shop's overall productivity.