CNC machines operate with micron-level precision, but this precision only becomes meaningful when the properties of the material being machined are suitable. Steel selection affects CNC machining performance through the following fundamental parameters:

Machinability: This refers to how easily and efficiently the steel can be machined with a cutting tool. High machinability allows for higher cutting speeds and longer tool life.

Hardness: As hardness increases, machinability decreases. However, a certain level of hardness is necessary for the strength of the final part. Ideal steel selection balances these two parameters.

Chip Form: Steels that produce short and brittle chips create fewer problems in CNC automation systems. Long and coiled chips can cause machine downtime and surface defects.

Dimensional Stability: Cold-drawn steels offer a narrower tolerance range compared to hot-rolled alternatives, providing predictable results in CNC machining.

Surface Quality: The internal structure and inclusion distribution of the steel directly affect the resulting surface roughness.

The operating conditions of the part are the primary factor determining steel selection. While S355JR or C45 are sufficient for parts subjected to static loads, high-strength heat-treatable steels such as 42CrMo4 or 34CrNiMo6 are required for parts exposed to dynamic and impact loads. If wear resistance is critical, carburizing steels (16MnCr5) are preferred.

The cylinder body tube must safely withstand the operating pressure and provide an excellent internal surface finish to ensure smooth piston movement. ST52 (E355+N) is the most commonly used seamless steel tube grade in the production of hydraulic cylinder bodies. After honing, the internal surface roughness is brought to the range of Ra 0.2–0.4 µm.

The wall thickness calculation for selecting a cylinder tube is based on the operating pressure and safety factor. The Barlow formula (P = 2St/D) is the basic calculation method; here, P is the operating pressure, S is the material’s yield strength, t is the wall thickness, and D is the outer diameter. A safety factor of between 3:1 and 4:1 is typically applied.

Die steels are a specialized subcategory of tool steels used in the forming processes of metal or plastic materials. Possessing properties such as high wear resistance, thermal fatigue resistance, hardness, and toughness, these steels differ fundamentally from general-purpose structural steels.

Die steels are classified into two main categories: cold-work tool steels (typically used in dies operating below 200°C) and hot-work tool steels (used in dies operating above 200°C, particularly in the 300–1200°C range). While plastic injection molds are generally classified under the cold work category, hot forging dies fall under the hot work category. In both categories, the raw steel material—whether round, square, or flat steel bars—is machined using CNC in mold manufacturing facilities to achieve the final mold shape.

The performance of die steels depends on the application of proper heat treatment. Pre-hardened steels (1.2738, 1.2312) are delivered with a pre-set hardness value and therefore do not require additional heat treatment. However, hardenable steels (1.2344, 1.2083, 1.2714) must undergo controlled quenching and tempering.

Heat treatment is particularly critical for hot-forging die steels. For example, to achieve optimal performance with 1.2344, austenitization at 1020–1040°C, followed by oil or air cooling and then two tempering cycles, is typically applied. The tempering temperature is selected between 520–620°C depending on the desired balance of hardness and toughness.

Important note: Dimensional changes occurring during heat treatment are directly related to CNC machining. The general approach for heat-treating die steels is: rough machining → heat treatment → finish machining.

Before starting material selection in a transmission shaft design, the load profile of the application, operating environment, and manufacturing requirements must be fully defined. As emphasized in Shigley’s Mechanical Engineering Design, shafts primarily operate under torsional and bending loads; the fatigue nature of these loads makes the material’s endurance limit a primary design parameter.

Mechanical Loads

Static and dynamic loads must be evaluated together. Peak torque values, sudden load changes, and vibration spectrum define the minimum requirements for yield strength (Rp0.2), tensile strength (Rm), and impact toughness (Charpy/ISO-V).

Fatigue and Surface Properties

Since shafts mostly operate under rotating bending loads, surface quality and hardness are of critical importance. In the Shigley approach, fatigue strength correction factors (surface, size, reliability, temperature, etc.) are applied to determine the working endurance limit. Therefore, surface hardening processes—such as carburizing, nitriding, or induction hardening—are an integral part of transmission shaft design.

Wear Resistance

In regions where the shaft surface contacts gears, bearings, or keys, high surface hardness (typically ≥ 58 HRC) is required. This level of hardness can only be achieved through carburizing + quenching of low-carbon alloy steels or nitriding of medium-carbon steels.

Maintaining Toughness

While achieving high surface hardness, it is essential to prevent brittleness in the core. Hardening depth and core alloy composition ensure this balance. Core toughness is especially critical for shafts operating under impact loads.

Machinability and Cost

The parameters determining cost and manufacturability are alloy content and post-heat-treatment hardness. For shafts with complex geometries, ease of machining prior to heat treatment becomes an important selection criterion.

Case carburizing is the process of enriching the surface of low-carbon steel in a carbon-rich environment (gas, solid, or plasma) at 900–950 °C, increasing the surface carbon content to the range of 0.7–1.0%. The subsequent quenching process creates a martensitic structure on the surface, while the core retains a tougher internal structure due to the low-carbon nature of the austenite. The EN 10084 standard defines the heat treatment conditions and property requirements for carburizing steels.

Effective case hardening depth (CHD) is determined according to the application loads and gear module. Typical CHD values range from 0.5–1.5 mm for automotive transmission shafts, while they can reach 2.0–3.5 mm for large industrial shafts.

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Hot-rolled steel is produced by rolling steel at high temperatures above its recrystallization temperature.
Cold-rolled steel, on the other hand, is obtained by re-rolling hot-rolled steel at room temperature.

This fundamental difference in production directly affects:

• surface quality,
• dimensional tolerances,
• mechanical strength,
• cost,
  and areas of application.

In short, the difference is not just a matter of “hot” or “cold” processing; the performance characteristics of the resulting product are completely different.

Cold-rolled steel is preferred in the automotive industry due to its surface quality and dimensional accuracy.

420 stainless steel is a type of steel belonging to the martensitic stainless steel group. This group includes stainless steels that can be hardened by heat treatment due to their high carbon content.

420 grade stainless steel primarily contains:

• Iron (Fe)
• Chromium (Cr)
• Relatively high carbon (C)

This composition gives the steel both corrosion resistance and high hardness. Especially when heat treated, 420 stainless steel becomes a very hard and durable material.

Like any material, 420 stainless steel has certain limitations:

• Its corrosion resistance is lower than that of austenitic stainless steels
• Its weldability is limited
• If improper heat treatment is applied, brittleness may increase

Therefore, its application area must be correctly determined.

Steel types are generally classified according to four main criteria:

• By production method

• By carbon content

• By alloy structure

• By intended use

This classification ensures that the right steel is used in the right application. Incorrect steel selection can lead to high costs, low performance, and long-term structural problems.

Hot rolling is the process of heating steel to a temperature range of 900–1200°C and then flattening it between rollers to increase its length and achieve the desired cross-sectional shape. During this process, the metal becomes more malleable due to the high temperature, enabling large-scale production.

The types of steel subjected to this process are called hot-rolled steels at the end of production and serve as a basic raw material in many areas of industry.

One of the most commonly used products in the production of pins, bolts, gear blanks, and fasteners is the hot-drawn round steel group. Thanks to its diameter accuracy, machinability, and wide range of dimensions, it is an ideal solution for both light machinery and heavy industrial projects.

Steel is a metal alloy obtained by alloying iron with carbon in certain proportions. The carbon content generally varies between 0.2% and 2.1%. This ratio has a directly determining effect on the hardness, strength, and formability of steel.

Pure iron is structurally soft and a metal with low strength. However, when combined with carbon, the resulting steel becomes much more durable, controllable, and suitable for industrial use. For this reason, steel is considered a developed and engineering-optimized form of iron.

One of the most important factors that make steel special is that its properties can be controlled during the production process. Thanks to the carbon ratio, alloying elements, and applied heat treatments, steels with very different performance characteristics can be produced from the same base material.

Steel is the main material of load-bearing systems. Earthquake-resistant structures, bridges, and high-rise buildings are made possible thanks to the strength of steel.