It is one of the industries with the highest expectations from steel. In modern vehicles, steel is used as a load-bearing element in the chassis and body structure, to provide high strength and wear resistance in engine and transmission components, in applications requiring fatigue strength such as suspension and steering systems, and for impact absorption in safety components.

Each application area requires different mechanical properties, surface qualities, and machinability characteristics. For example, while high yield strength and good weldability are essential for a chassis cross member, hardness, wear resistance, and tight tolerances are critical for a gear shaft.

In recent years, with the lightweighting trend, the use of high-strength steels (AHSS – Advanced High Strength Steel) has been increasing rapidly. These steels provide the same load-bearing capacity with thinner sections, contributing to reduced vehicle weight and improved fuel efficiency.

Standardized according to European Norms (EN). A separate EN standard applies to each profile form:

In Turkey, the steel designations St37 and St52, still widely used, belong to the old DIN standard classification originating from Germany. Today, the EN 10025 standard, valid throughout the European Union, has replaced these designations. However, the old designations are still frequently used in the field, workshops, and ordering processes.

In the S235 and S355 designations, the “S” stands for “Structural,” while the number next to it indicates the minimum yield strength of that steel in MPa (Megapascals). Thus, the minimum yield strength of S235 is 235 MPa, and that of S355 is 355 MPa.

Choosing between S235 (St37) and S355 (St52) is not a question with a single right answer. The correct choice is made by considering the loads the structure will be subjected to, welding requirements, budget, forming needs, and design codes together.

As a general rule: S235 should be preferred for lightweight structures, ease of processing, and budget priority; S355 should be preferred for high strength, heavy loads, and cross-section optimization.

At Uyar Çelik, with our half-century of experience, we manufacture and supply structural steel in accordance with the EN 10025 standard. For detailed information about our hot-rolled and cold-drawn steel bars produced in our facilities in Istanbul, Kocaeli, and Karabük, and for technical consultancy and price quotes, please contact us.

An ideal steel supplier should be able to offer hot-rolled (round, square, flat bar, hexagonal), cold-drawn (bright) bars, specialty steels (heat-treated, carburized, free-machining steels), and imported special alloy steels under one roof.

For example, a machine manufacturer might need hot-rolled S355JR square bar for chassis, cold-drawn C45 round bar for shafts, and cold-drawn 16MnCr5 round bar for gears. Being able to source these three different grades and forms from a single supplier simplifies logistics coordination, streamlines invoice management, and provides the advantage of bulk purchasing.

Heat treatment refers to thermal processes applied to steel to alter its internal structure (microstructure). Steel transforms into different crystal structures at different temperatures; these transformations are controlled to obtain the desired mechanical properties.

Basic Purposes of Heat Treatment

To increase hardness: For parts requiring wear resistance (gears, shafts, molds)
To increase strength: To provide higher load-carrying capacity
To improve toughness: To increase impact resistance by reducing brittleness
To facilitate machinability: To prepare for machining or cold forming
To relieve internal stresses: To reduce stresses that occur after welding, rolling, or forging
To homogenize the microstructure: To obtain consistent properties by smoothing the grain structure

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.