by Rolf Olofsson
What do an ancient Roman war strategy and ISO P steel turning grades have in common, and how can this help increase your machine shop’s output? Here, Rolf Olofsson, product manager at Sandvik Coromant explores how the coating and substrate of carbide grades can make a huge difference in the efficiency and productivity of steel turning processes.
A common misconception in the metalworking industry is that machining steel is simple. Experienced machinists know that turning ISO P steel is anything but. First among many concerns is the breadth of materials in the ISO P classification, which range from ductile low-carbon steels to high-alloyed.
Secondly, the hardness of different steels ranges significantly from one end of the spectrum to the other. The type of application varies – as do machining conditions in workshops.
Clearly, 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. So too is a cutting edge capable of delivering the hardness needed to resist plastic deformation induced by the extreme temperatures present in the cutting zones.
What’s more, the grade must be equipped with a coating that can prevent flank wear, crater wear and edge buildup. It’s also vital that the coating adhere to the substrate; if it doesn’t stick, the substrate is exposed, leading to rapid failure.
Given this array of demands, it’s crucial to understand the structure of a steel turning grade in order to make an informed decision when picking one for your application.
Structure of a carbide insert
All carbide grades contain a cemented carbide core, also known as a substrate. The substrate defines the toughness and strength of the grade. It also determines the grade’s resistance to plastic deformation.
The cemented carbide substrate is usually covered by a few layers of coating such as titanium carbonitride (TiCN), alumina (Al2O3), or titanium nitride (TiN), which give the insert its edge toughness, adhesion and wear resistance properties. The recipe for superior resistance to different kind of wears, flank, crater and edge buildup, adhesion to substrate and improved tool life also lies in the microscopic details that go into designing the coating layer.
Roman shield wall
In conventional alumina coating, crystal growth direction is random, as depicted in Figure 1a. If the growth in the coating layer can be controlled to ensure all crystals line up in the same direction, as depicted by yellow in Figure 1b, superior wear resistance is the result.
To help you understand the power of crystal alignment, let’s consider an example from Roman history. When Roman legions conducted a siege, they frequently deployed a shield wall — the testudo (“tortoise”) formation. In this formation all shields were aligned and tightly packed, eliminating vulnerable gaps. The shield wall helped the Romans resist attack while advancing.
The alignment of crystals in a coating layer works similarly. 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 all crystals line up in the same direction, with the strongest part towards the top surface. This patented technology, known as Inveio coating, gives inserts a new level of wear resistance and tool life.
The tightly packed unidirectional crystals create a strong barrier oriented towards the cutting zone and chip. This helps Inveio equipped grades greatly improve resistance to crater wear and flank wear. Another benefit is that heat is more rapidly led​ 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 — featured in Sandvik Coromant’s latest ISO P steel turning grades GC4415 and GC4425 — the benefits of unidirectional coating have been enhanced. Improved crystal orientation makes for 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 requirement as it helps avoid any sudden insert breakages.
Look for inserts that have undergone post-treatment: a process in which very fine, sharp ceramic particles are bombarded onto the insert coating. Imagine a hammer striking on the coating to reinforce and strengthen it. Inserts that have undergone effective post-treatment perform well during intermittent cutting.
The new GC4415 and GC4425 ISO P steel turning grades are ideal for manufacturers operating in mass and batch production setups. Equipped with a new substrate at their heart that is reinforced by Inveio technology, the two grades offer reliable performance and superior wear resistance. In addition, their new post-treatment boosts performance in intermittent cutting operations, avoiding sudden breakages and enabling both insert grades to outperform over a broad application range.
Customers have been able to implement higher cutting speeds (Vc) and multiplied feed rates (Fn) with these grades. A general engineering manufacturer subjected a 4140 pre-heat treated steel workpiece to multidirectional external roughing with the GC4425 insert. Compared with using a competitor’s ISO insert for the same process, the customer was able to achieve a 100 per cent productivity increase, with a reduced cycle time of 50 per cent – plus a 30 per cent cost reduction.
Machining ISO P steel is tricky. By keeping a few considerations in mind when selecting a grade – such as substrate toughness and new advances in the area of material science and tooling technology – you can make a huge difference in your steel turning efficiency and your machine shop’s overall productivity. SMT
