The Influence of Different Thicknesses of Medium Thick Carbon Steel Plates

Defining Medium Thick Carbon Steel Plates: Thickness Classification and Material Grades

Medium thick carbon steel plates occupy a critical position in the structural steel spectrum, bridging the gap between thin sheets and extra-heavy plates. By industry convention, medium thick plates are defined as those with thicknesses ranging from 4.5mm to 25mm, while plates from 25mm to 100mm are classified as thick plates, and those exceeding 100mm as extra-thick plates. Within the medium thick category, common grades include Q235B, a carbon structural steel with a yield strength of at least 235 MPa and tensile strength of 370–500 MPa, and Q355B, a higher-strength grade offering a minimum yield strength of 355 MPa and tensile strength of 470–630 MPa. The Q355B grade incorporates microalloying elements such as niobium, vanadium, and titanium, which contribute to grain refinement and precipitation strengthening, making it more suitable for heavy-duty structures requiring high load-bearing capacity and ductility. The thickness of these plates fundamentally influences their mechanical behavior, fabrication characteristics, and ultimately their suitability for specific engineering applications.

Thickness and Mechanical Properties: The Yield Strength Reduction Effect

One of the most significant effects of plate thickness on medium thick carbon steel is the systematic reduction of yield strength as thickness increases. For hot-rolled carbon steels, the nominal yield strength quoted in material designations is typically the value for material up to 16mm thick. As plate or section thickness increases beyond this threshold, the minimum specified yield strength decreases. This phenomenon occurs because thinner material receives greater mechanical working during rolling, which increases strength through work hardening. For Q235B steel, yield strength varies with thickness: 235 MPa for thicknesses up to 16mm, 225 MPa for 16–40mm, 215 MPa for 40–100mm, and 195 MPa for 100–150mm. Regression analysis of Q345B steel has confirmed that both yield strength and tensile strength decrease with increasing plate thickness, while the influence of rolling specifications on yield strength is greater than on impact tensile strength. Studies have shown that yield strength can fall by approximately 50 N/mm² as gauge increases, attributed to a combination of coarser grain size, lower interstitial levels, and reduced pearlite contribution to yield strength arising from slower cooling rates. For ASTM A36 steel, the yield value similarly decreases with increasing material thickness. This thickness-dependent strength reduction must be carefully accounted for in structural design to ensure that the actual capacity of thicker plates meets the required safety margins.

Thickness, Microstructure, and Toughness: The Challenge of Thick Sections

The influence of plate thickness extends beyond strength to affect microstructure, toughness, and the risk of brittle fracture. In thicker plates, cooling rates during and after rolling vary significantly through the thickness—the surface cools rapidly while the core cools more slowly. This differential cooling leads to microstructural variations across the plate section, with coarser grain structures typically developing in the slower-cooling core regions. The toughness of steel and its ability to resist brittle fracture decreases with increasing thickness, tensile stress, stress raisers, and at colder temperatures. For plates exceeding 50mm in thickness, toughness is remarkably lowered, necessitating additional measures in heat treatment and steel composition determination to ensure adequate performance in actual steel constructions. The transition from high to low toughness as temperature is lowered is highly dependent on material thickness. In practical terms, this means that thicker medium carbon steel plates require more careful consideration of service temperature, impact loading conditions, and potential stress concentrations. For structural applications in cold climates or where dynamic loading is anticipated, specifying normalized or thermomechanical controlled processed (TMCP) plates can help mitigate the toughness reduction associated with increased thickness.

Thickness and Weldability: Preheat Requirements and Heat-Affected Zone Control

The thickness of medium carbon steel plates has a direct and significant impact on welding procedures, particularly regarding preheat requirements and heat-affected zone (HAZ) characteristics. As plate thickness increases, the thermal diffusivity changes, resulting in faster cooling rates in the HAZ. This rapid cooling can lead to the formation of hard, brittle martensitic microstructures in the HAZ, increasing the susceptibility to hydrogen-induced cold cracking. To prevent such cracking, preheating is essential for thicker plates. For low-carbon steel with carbon content of 0.21–0.28%, plates up to 16mm thickness may be welded without preheat at ambient temperatures above -10°C, while plates exceeding 16mm require preheat of 100–200°C under the same conditions. General guidelines specify that when carbon equivalent (CE) is below 0.4% and plate thickness is less than 25mm, preheat may be omitted; when CE is between 0.4% and 0.6% and thickness exceeds 25mm, preheat of 100–200°C is required; and when CE exceeds 0.6%, preheat temperatures of 200–370°C are recommended. The minimum preheat temperature should be at least 175°F (approximately 80°C) for medium-carbon steels, with maximum interpass temperature not exceeding 400°F (approximately 204°C). For S35C steel plate, preheating to 150–250°C is typically required depending on thickness. Additionally, thicker plates welded with narrow-groove or V-groove configurations face the problem of molten pool sagging due to gravity, which can lead to defect formation and deteriorate mechanical properties. Proper joint design, controlled heat input, and post-weld heat treatment are therefore essential for achieving reliable welds in medium thick carbon steel plates.

Thickness and Cutting Technologies: Selecting the Optimal Method

The choice of cutting technology for medium thick carbon steel plates is heavily influenced by plate thickness, with different methods offering distinct advantages across the thickness range. For thinner medium plates (up to 6mm), fiber laser cutting delivers exceptional speed and edge quality, with cutting speeds reaching 10,000 mm/min for carbon steel up to 6mm thickness. However, as thickness increases beyond 10mm, high-definition plasma cutting becomes increasingly competitive, with speeds reaching 1.23 meters per minute for low-carbon steel thicker than 10mm, exceeding the performance of standard-power lasers. For plates in the 10–25mm range, plasma cutting remains a viable and efficient option, with cutting speeds of 800–1000 mm/min. Hybrid laser-flame cutting technology offers an interesting alternative for 10mm thick low-carbon steel, achieving the same cutting speed as a 12kW pure laser (3.5 m/min) with lower power consumption and superior edge verticality (taper less than 1°). Oxy-fuel cutting remains a cost-effective solution for thicker plates where edge quality requirements are less demanding. The selection of cutting method must balance speed, edge quality, capital equipment cost, and operating expenses based on the specific thickness range and production volume of the application.

Application Fields: Thickness-Driven Selection Across Industries

The diverse thicknesses of medium thick carbon steel plates serve distinct application domains across multiple industries. Medium plates (4.5–25mm) are predominantly used in construction engineering, machinery manufacturing, container fabrication, shipbuilding, and bridge construction. Within this range, thinner medium plates find application in automotive frame steels, boiler plates, and welded structural components, while thicker medium plates are specified for pressure vessels, ship hulls, and heavy machinery frames. For bridge construction, medium plates must withstand dynamic loads, impact, vibration, and corrosion. Shipbuilding plates require high strength, plasticity, toughness, cold-bending performance, weldability, and corrosion resistance. Boiler plates operate under medium-temperature (below 350°C) and high-pressure conditions, requiring guaranteed strength, good welding, and cold-bending properties. Pressure vessel plates, operating at pressures from atmospheric to 630 kg/cm² and temperatures from -20°C to 450°C, demand not only strength and good plasticity and toughness but also excellent cold-bending and welding performance. The selection of the appropriate thickness for each application must consider not only the mechanical load requirements but also the fabrication processes involved, the service environment, and the economic constraints of the project.