1. Ordinary carbon structural steel
Q195 steel has low strength, good plasticity, toughness, processing performance and welding performance, and is mainly used for rolling thin plates and wire rods.
Q215 steel is mainly used to make pipe billets, bolts, etc..
Q235 steel has moderate strength, good load-bearing capacity, good plasticity and toughness, good weldability and processability, and is a commonly used grade for steel structures. It is made into steel bars, steel sections and steel plates in large quantities for building houses and bridges.
Q275 steel has high strength and hardness, good wear resistance, but poor plasticity, impact toughness and weldability. It is mainly used to manufacture shafts, agricultural tools, wear-resistant parts and pads.

2. Fine carbon structural steel, fine carbon structural steel is carbon steel with a carbon content of less than 0.8%. This steel contains less sulfur, phosphorus, and non-metallic inclusions than carbon structural steel. Compared with ordinary carbon structural steel, high-quality carbon structural steel has higher plasticity and toughness, and can be strengthened by heat treatment. It is mostly used for more important parts and is widely used in machinery manufacturing.

According to the different carbon content, high-quality carbon steel is divided into low carbon steel, medium carbon steel and high carbon steel. Low carbon steel has low strength and hardness, but high plasticity and toughness, excellent processing and welding properties, and is used to manufacture parts with small loads and high toughness requirements, as well as small carburized parts; medium carbon steel has high strength and hardness, low plasticity and toughness, good cutting performance, but poor welding performance, good cold and hot deformation ability, and is mainly used to manufacture mechanical parts with large loads. Commonly used medium carbon steels are 40#, 45# and 50# steels, but because the hardenability of carbon steel is not high, the larger the size of the part, the worse the strengthening effect of the quenching and tempering treatment.

316 stainless steel belongs to austenitic stainless steel. The key to high temperature resistance is the addition of 2.0% to 3.0% Mo element. The presence of this element enables 316 stainless steel to maintain stable chemical and mechanical properties under high-temperature environments.

The temperature resistance of 316 stainless steel pipes shows significant environmental dependence:
Oxidizing environment: It is recommended that the temperature for long-term continuous use should not exceed 800°C; otherwise, the oxide film may fail due to continuous oxidation.
Non-oxidizing environment: Under anaerobic or low-oxygen conditions, it can withstand high temperatures of around 1200°C, and even 1300°C for short-term intermittent use.
Load softening temperature: About 950°C, that is, the critical temperature at which the material begins to soften under stress conditions.

The range of high temperatures that 316 stainless steel can withstand is roughly 1200-1300 degrees, and it can be used under relatively harsh conditions. The reason why 316 stainless steel can withstand such high temperatures is that the addition of Mo element to 316 stainless steel greatly increases its corrosion resistance and high temperature strength.

After quenching, but before tempering, the hardness of 45 steel is greater than HRC55 (up to HRC62) to be qualified.
The highest hardness in practical application is HRC55 (HRC58 for high-frequency quenching).

Quenching hardness:
Quenching is to change the internal structure of steel by rapid cooling, thereby increasing its hardness and strength. For 45 steel, the hardness after quenching can usually reach HRC55-58, and the limit value can even reach HRC62. However, this requires specific conditions to achieve, such as appropriate heating temperature, holding time and quenching cooling rate. It is worth noting that the hardness of 45 steel after quenching may be higher before tempering treatment, so in practical applications, the qualified standard is usually set to a hardness greater than HRC55.
Quenching and tempering hardness:
Unlike quenching, quenching and tempering treatment aims to eliminate the internal stress generated by quenching through tempering, so that the steel can maintain a certain hardness while improving its toughness and plasticity. After quenching and tempering treatment, the hardness range of 45 steel is usually between HRC20-HRC30. This hardness range enables 45 steel to have a certain load-bearing capacity and wear resistance while maintaining good processing performance.