Risk analysis of hammer head fracture in hammer crusher: mechanism and prevention of head detachment accidents caused by insufficient toughness

The head detachment or overall fracture of the hammer head of a hammer crusher during operation is a high-risk sudden failure. This type of accident can result in minor damage to the sieve plate and casing, severe damage to the rotor shaft, cracking of the bearing seat, and even personal safety accidents. In depth analysis shows that among various factors, insufficient toughness of the hammer material is the core intrinsic factor leading to brittle fracture.

1、 Accident characteristics and direct consequences

Hammer head fracture accidents usually manifest in two forms:

Head detachment: The striking end of the hammer (the most severely worn area) breaks along the neck or internal defect, separating from the handle.

Whole fracture: The hammer head completely breaks into two or more pieces along the hammer hole, handle, or middle.

The direct consequences are extremely serious:

Serious equipment damage: The fragments flying out at high speed have enormous kinetic energy, which is enough to penetrate the casing lining plate, destroy the sieve plate (grate plate), and cause secondary impact damage to the inner wall and rotor body of the crusher.

Long term production interruption: The accident cleanup is complex, and replacing damaged parts is time-consuming and labor-intensive, resulting in unplanned long-term downtime.

The safety risk is enormous: if debris flies out of the aircraft, it poses a fatal threat to on-site personnel.

2、 Fracture mechanism: brittle failure under insufficient toughness

The hammer strikes the material at high speed, bearing high-energy and high-frequency impact loads. Resilience is the ability of a material to absorb plastic deformation work and impact energy before fracture. When the toughness is insufficient, the material's resistance to cracks sharply decreases, and the failure mode shifts from safe plastic wear to dangerous brittle fracture. The fracture process follows the following steps:

crack initiation

Stress concentration point: The transition fillet on the hammerhead structure is too small, key slots, screw holes, or deep grooves and sharp edges formed after wear, which can generate extremely high local stress under impact loads.

Microscopic defects: Casting defects inside the material (such as pores, shrinkage porosity, slag inclusions), coarse carbides, non-metallic inclusions, etc., become natural sources of microcracks under stress.

Impact overload: When encountering unbreakable objects (such as iron blocks) or large materials, the impact stress instantly exceeds the design value.

crack propagation

Once a crack is initiated, significant stress concentration will occur at the crack tip under subsequent periodic impact loads.

Materials with sufficient toughness, such as tough austenite, can passivate crack tips through plastic deformation, consume energy, and thus prevent or delay crack propagation.

Materials with insufficient toughness, such as some brittle high chromium cast iron or poorly heat-treated structures, have almost no plastic deformation at the crack tip, and cracks will rapidly and unstably propagate along grain boundaries or through hard phases.

Unstable fracture:

When the crack extends to a critical size (depending on the material's fracture toughness and stress level) and the remaining section cannot withstand the working load, the hammer head will undergo instantaneous brittle fracture. The fracture surface is usually smooth macroscopically, radiating or crystalline, and brittle features such as river patterns can be seen microscopically.

3、 The key factor leading to insufficient toughness of the hammer head

Types of influencing factors, specific reasons and manifestations

Improper material selection, in pursuit of high hardness and wear resistance, chose materials with low fracture toughness values (such as certain types of high chromium cast iron), without fully considering the impact energy level of the working conditions. Insufficient high-temperature oxidation resistance or corrosion resistance of materials can lead to performance degradation under specific working conditions.

Manufacturing process defects and casting defects: severe weakening of effective cross-sectional area and stress concentration caused by porosity, shrinkage, slag inclusion, etc.

Improper heat treatment: Excessive quenching temperature or rapid cooling can lead to excessive tissue stress and the formation of microcracks; Insufficient tempering leads to the failure to eliminate internal stress and high brittleness of the structure.

Poor microstructure: Carbides are too coarse or distributed in a continuous network, severely cutting the matrix and becoming a convenient channel for crack propagation.

The structural design is unreasonable, and the transition fillet radius at the connection between the head and handle of the hammer is too small, resulting in a high stress concentration factor. Unreasonable design of the worn end shape can easily lead to the formation of fatigue crack sources in specific areas.

Operating beyond the design capacity under working conditions: Long term handling of hard and oversized materials, or excessive rotor linear speed, causing the hammer head to withstand impact energy beyond its material tolerance range.

Unforeseeable impact: instantaneous extreme overload caused by the failure of the iron removal system and the entry of metal foreign objects into the crushing chamber.

Corrosion environment: Stress corrosion can significantly reduce the actual toughness of materials and promote crack initiation during wet operations or crushing of materials containing corrosive components.

4、 Systematic Prevention Strategy and Quality Control

Preventing brittle fracture of hammer heads is a systematic project that requires full control from the source to use.

Scientific selection and applicability verification

Matching impact energy: Based on the hardness, block size, and rotor speed of the crushed material, estimate the single impact energy and select a material with an appropriate impact toughness (AKV value). For high impact working conditions, sufficient toughness should be prioritized, and high toughness alloy steel, ultra-high manganese steel, or bimetallic hammer heads composite with high wear-resistant materials on a strong toughness matrix can be selected.

Introduction of toughness index: In the procurement technical agreement, not only hardness (HRC) is specified, but also the minimum requirements for impact toughness at room temperature and working temperature must be clearly defined and used as the basis for acceptance.

Strengthen manufacturing process and factory quality control

Process control: Suppliers are required to use advanced casting processes such as refining and modification to ensure the density of castings. Implement standardized heat treatment process curves and conduct sufficient stress relief tempering.

Non destructive testing: For large and critical hammer heads, ultrasonic testing (UT) or magnetic particle testing (MT) may be required to detect internal and surface defects.

Sampling destructive testing: Regular sampling is conducted for mechanical property testing (hardness, impact toughness) and metallographic structure inspection to ensure that material properties and structure meet the requirements.

Standardized use, maintenance, and monitoring

Overloading operation is strictly prohibited: strictly control the feeding particle size and iron removal to avoid the entry of unbreakable materials.

Regular inspection and replacement: Establish a regular inspection system for hammer heads, focusing on checking the surface of the hammer head (especially the neck and worn end) for visible cracks. Once cracks are found, they must be replaced immediately. Adopting the principle of symmetrical group replacement to maintain rotor balance.

Predictive replacement: Even if no cracks are found, when the hammer head wears to a certain proportion of its initial weight (such as a loss of 30%) or the remaining neck thickness significantly decreases, preventive replacement should be considered because the stress on the remaining section has increased.

Vibration monitoring: Install an online vibration monitoring system. When the rotor is unbalanced or the hammer head cracks, the vibration value will change, which can provide early warning.