Heat treatment of hydraulic cylinder

Heat treatment is a crucial step in the manufacturing of hydraulic cylinders, directly affecting the durability, reliability and service life of the cylinders.

1. Why is heat treatment necessary for hydraulic cylinders?

During operation, hydraulic cylinders are subjected to significant internal pressure, external loads, impacts and friction. Their main components (especially the cylinder barrel and the piston rod) must possess the following properties:

液压油缸在工作中承受着巨大的内部压力、外部负载、冲击和摩擦。其主要部件（尤其是缸筒和活塞杆）必须具备以下性能：

1. High strength and rigidity: Resisting expansion and deformation caused by pressure.

1. 高强度和刚度：抵抗压力引起的膨胀和变形。
2. High wear resistance: Reducing wear between the piston seal and the inner wall of the cylinder, as well as between the piston rod and the guide sleeve/seal.

2. 高耐磨性：减少活塞密封件与缸筒内壁、活塞杆与导向套/密封件之间的磨损
3. Good fatigue resistance: Withstanding alternating stresses brought by reciprocating motion to prevent fatigue fracture.

3. 良好的耐疲劳性：承受往复运动带来的交变应力，防止疲劳断裂。
4. Certain toughness: Resisting impact loads to prevent brittle fracture.

4. 一定的韧性：抵抗冲击载荷，防止脆性断裂

The raw materials (usually carbon steel or alloy steel) often cannot simultaneously meet these demanding requirements when in their supplied states (such as annealed or rolled states).

Heat treatment is a key process that achieves the desired mechanical properties by controlling the heating and cooling of the steel, thereby altering its internal microstructure (metallographic composition).

原材料（通常是碳钢或合金钢）在供货状态（如正火或轧制态）下，往往无法同时满足这些苛刻的要求。 热处理就是通过控制钢材的加热和冷却过程，改变其内部微观结构（金相组织），从而获得所需力学性能的关键工艺。

Heat treatment of the piston rod
The piston rod is an exposed moving part. It needs to be wear-resistant, corrosion-resistant and resistant to bending.
· Common materials: 45# steel, 40Cr, 35CrMo, stainless steel (such as 2Cr13)
· Main heat treatment processes: quenching and tempering + surface hardening (or chrome plating)

活塞杆是外露的运动部件，既要耐磨，又要抗腐蚀、抗弯曲。

· 常用材料：45号钢， 40Cr， 35CrMo， 不锈钢（如2Cr13）

· 主要热处理工艺：调质 + 表面淬火（或镀铬）

Surface hardening (high-frequency/middle-frequency induction hardening): The outer surface of the piston rod is subjected to hardening to achieve a hardness of HRC 45-55, enhancing its wear resistance. After hardening, low-temperature tempering is also required.
·

Hard chrome plating: This is the most mainstream and classic process. On the base material of the piston rod that has undergone quenching or surface hardening, a hard chrome layer with a thickness of approximately 0.03-0.05mm is electroplated.

The hardness of the hard chrome layer is extremely high (HV 800-1000, approximately HRC 65-70), extremely wear-resistant, and has excellent corrosion resistance. Before plating, it is usually necessary to fine-grind, and after plating, polishing is required.

表面淬火（高频/中频感应淬火）：对活塞杆外圆表面进行淬火，使其硬度达到HRC45-55，提高耐磨性。淬火后同样需要低温回火。

· 镀硬铬：这是最主流、最经典的工艺。在经过调质或表面淬火的活塞杆基体上，电镀一层厚度约0.03-0.05mm的硬铬层。硬铬层硬度极高（HV800-1000，约HRC65-70），极其耐磨，并且具有优异的耐腐蚀性。镀铬前通常需要精磨，镀铬后需要进行抛光。

Cylinder tube manufacturing process:
Cutting → Rough machining → Quenching treatment → Semi-finish machining → Inner hole induction hardening → Low-temperature tempering → Finish machining (grinding) → Cleaning and assembly

缸筒制造流程：

下料→ 粗加工 → 调质处理 → 半精加工 → 内孔感应淬火 → 低温回火 → 精加工（珩磨） → 清洗装配

Piston rod manufacturing process:
Cutting → Rough machining → Quenching treatment → Semi-finish machining → Surface induction hardening → Low-temperature tempering / or directly selected → Hard chrome plating → Finishing (grinding, polishing) → Cleaning and assembly

活塞杆制造流程：

下料→ 粗加工 → 调质处理 → 半精加工 → 表面感应淬火 → 低温回火 / 或者直接选择 → 镀硬铬 → 精加工（磨削、抛光） → 清洗装配

 Common Problems and Solutions
1. Insufficient hardness
· Reasons: Insufficient quenching temperature, insufficient cooling speed, excessive tempering temperature or prolonged tempering time.
· Effects: Poor wear resistance, early wear and cracking of the cylinder.

硬度不足

· 原因：淬火温度过低、冷却速度不够、回火温度过高或时间过长。

· 影响：耐磨性差，油缸易早期磨损、拉缸。

2. Deformation and bending
· Reasons: Uneven heating or cooling, improper release of internal stress in the material, unreasonable clamping method.
· Effects: Causes the cylinder to get stuck and leak, abnormal wear of the sealing components.

变形与弯曲

· 原因：加热或冷却不均匀、材料内部应力释放不当、装夹方式不合理。

· 影响：导致油缸卡滞、泄漏，密封件异常磨损。

3. Quenching cracks
· Reasons: Excessive cooling speed (especially for high-carbon steel or complex-shaped parts), defects in the material (such as inclusions), failure to perform tempering promptly after quenching.
· Effects: Directly leads to the destruction of the part, and even causes cracking under pressure.

淬火裂纹

· 原因：冷却速度过快（尤其对于高碳钢或复杂形状件）、材料本身有缺陷（如夹杂物）、淬火后未及时回火。

· 影响：直接导致零件报废，甚至在压力下发生爆裂。

4. Eroded chromium coating
· Reasons: Incomplete surface treatment of the base material, improper pre-treatment before plating, insufficient hardness of the base material or excessive thickness of the coating.
· Effects: Rapid rusting and wear of the piston rod, contamination of the hydraulic system.

镀铬层脱落

· 原因：基体表面处理不干净、镀前处理不当、基体硬度不足或镀层过厚。

· 影响：活塞杆快速锈蚀、磨损，污染液压系统。

Heat treatment is the core technology that gives hydraulic cylinders their "steel-like structure".

By performing quenching and tempering, a strong and tough matrix is obtained. Through surface hardening or chrome plating, a hard and wear-resistant surface is achieved.

This combination of "outer hardness and inner toughness" ensures that hydraulic cylinders can operate stably and persistently under various harsh conditions.

When choosing a hydraulic cylinder, understanding the heat treatment process and level of its key components is an important basis for judging the quality of the product.

热处理是赋予液压油缸“钢筋铁骨”的核心技术。通过调质获得强韧的基体，通过表面淬火或镀硬铬获得坚硬耐磨的表面，这种“外硬内韧”的组合，确保了液压油缸能够在各种严苛的工况下稳定、持久地工作。在选择液压油缸时，了解其关键部件的热处理工艺和水平，是判断其质量高低的重要依据。