Cemented carbide is a powder metallurgy product primarily composed of micron-sized tungsten carbide powder, a high-hardness refractory metal, with cobalt, nickel, or molybdenum as binders. It is sintered in a vacuum furnace or hydrogen reduction furnace. The production process of cemented carbide is highly complex, and even minor oversights can easily lead to defects. Today, I will share with you the common types of cemented carbide defects and their causes.

 

Contamination

Characteristics: Contamination is characterized by the presence of unevenly sized pores inside the cemented carbide product, with corresponding surface protrusions or holes.

If the surface is slightly contaminated and can be machined without leaving holes, the product can be considered qualified and released.

If the surface is severely contaminated or exhibits blistering, it should be classified as scrap.

Causes of Contamination

During the high-temperature sintering stage, gases generated by internal reactions in the sintered body escape or migrate to the surface. By this time, the liquid phase has already begun to solidify, leaving behind small pores that cannot recover in time, and the gases migrating to the surface are not completely expelled.

Certain difficult-to-reduce oxides are only reduced at the temperature where the liquid phase forms. The pressure of the gases produced by reduction exceeds the resistance of the liquid phase contraction, leading to blister formation.

1.Excessive temperature (over-sintering) causes a significant increase and aggregation of the liquid phase, resulting in blistering.

2.Impurities in the pressed blocks, such as carbide chips or copper wires, can also cause blistering (contamination).

3.Severe delamination in the pressed product can also manifest as blistering during sintering.

Sources of Contamination

1.Oxidized block materials, oxidized granular materials, and defective pressed blanks.

2.Metal impurities: Screen mesh debris, cobalt chips.

3.Non-metal impurities: Ceramic fragments, glass fragments, boat-filling materials, dust, brush debris, etc.

4.Forming agents: Unremoved mechanical impurities, unfiltered gel, uneven forming agents, aged forming agents, etc.

Deformation

Characteristics: The geometric shape of the carbide product undergoes irregular changes, and warped products exhibit a regular curved deformation on a specific plane.

For such deformedcarbide products, inspections should be conducted according to standards or product drawings. Products that exceed tolerance limits should be returned to the production unit for reprocessing, and those that cannot be reprocessed should be classified as scrap.

Causes of Deformation Defects

1.Uneven density of the pressed product: This leads to uneven shrinkage during sintering. Areas with higher density shrink less, while areas with lower density shrink more.

2.Uneven carbon atmosphere around the pressed blank: This causes deformation of the product.

3.Uneven temperature environment during sintering: The pressed blank deforms due to temperature inconsistencies in the sintering environment.

4.Other reasons: Improper loading of the sintering boat, uneven placement of the base plate, etc.

Peeling

Characteristics: Peeling is characterized by the appearance of irregular branch-like cracks, cracks, or flaking at the edges and corners of the alloy product. In mild cases, it presents as a network of cracks, while in severe cases, small pieces may peel off. In extreme cases, the product may crack and peel off entirely, with cotton-like carbon black deposits clearly visible at the peeling sites. Carbide products with peeling are directly classified as scrap.

Causes of Peeling

1.High concentration of carbon-containing gases in the low-temperature zone: High concentrations of carbon-containing gases penetrate weak areas of the product (such as edges and corners, which often have lower density or significant elastic aftereffects). Under the catalytic action of cobalt, carbon precipitation reactions occur:

CH=C+H 2

CO =?C+CO

The precipitated carbon disrupts the continuity of the carbide, leading to peeling. In other words, the decomposition of carbon-containing atmospheres into large amounts of free carbon is the primary cause of peeling.

2.Vacuum dewaxing stage: If the dewaxing temperature exceeds 400°C (typically 375°C), it reaches the pyrolysis temperature of paraffin, generating low-molecular-weight paraffin, olefins, and free carbon. As the temperature continues to rise, paraffin pyrolysis intensifies. At this stage, the sintered body becomes porous and loose, significantly reducing its strength and making it difficult to withstand the impact of hydrocarbon gases generated by paraffin pyrolysis, leading to peeling.

Process Parameters Affecting Peeling

(1) Boat pushing speed and heating rate in the low-temperature zone

(2) Moisture content in hydrogen

(3) Loading amount in the boat

(4) Catalytic effect of cobalt

 

Carburization

Carburized carbide products have a shiny, oily black surface, with fine graphite dots or nest-like spots visible on the cross-section. In severe cases, the product may feel lubricated to the touch and leave black marks. Carburization generally affects the performance of the product and should be evaluated based on the specific grade and intended use. Non-compliant products should be returned to the production unit for reprocessing.

Causes of Carburization

1.Excessive total carbon content in the mixture

2.High carbon content in the filler material

3.High concentration of hydrocarbons in the low-temperature zone atmosphere

4.Diffusion of carbon from graphite boats into the sintered body

Rapid heating rate and short duration during the removal of the forming agent, causing the forming agent to decompose and generate free graphite, leading to carbide carburization

Sources of Free Carbon

1.Decomposition of the forming agent during the dewaxing (degumming) process

2.Diffusion of carbon from graphite boats

3.Control of the sintering atmosphere in the vacuum furnace

Decarburization

Decarburized carbide products exhibit white bright spots or shiny streaks on the surface, with silver-white shiny spots or tadpole-shaped pits visible on the fracture surface. The microstructure may show the presence of the η phase. Decarburization generally affects the performance of the product, and decarburized carbide products should be returned to the production unit for reprocessing.

Causes of Decarburization

1.Decarburization reaction during hydrogen sintering

The reaction between WC in the product and H? generates CH?. This reaction occurs throughout the sintering process and intensifies as the temperature rises.

At the furnace entrance, before complete shrinkage, decarburization occurs both internally and externally in the product.

At the furnace exit, after the product has shrunk, decarburization occurs on the surface. The intensity of the reaction depends on the flow rate of H?. The CH? generated by this reaction decomposes at high temperatures, causing carburization of the product.

Moisture in the furnace atmosphere reacts with WC or C at temperatures above 825°C:

H2O+WC→W+H2+CO

H2O+C→CO+H2

This reaction also occurs at both the entrance and exit of the furnace. Before complete shrinkage, it causes internal and external decarburization, while at the furnace exit, it causes surface decarburization.

Decarburization reaction during vacuum sintering

The deoxidation reaction during vacuum sintering occurs because the pressed blank contains oxygen, which is reduced by free carbon and carbon in WC during sintering. The reactions are:

MeO+C→Me+CO

MeO+2C→MeC+CO

This reaction also occurs at both ends of the furnace entry and exit. Before complete contraction, the U-shaped product causes decarburization both inside and outside. At the exit end, it causes decarburization on the product’s surface.

2.Vacuum sintering decarburization reaction

The deoxidation reaction after vacuum sintering occurs because the compact contains oxygen, which is reduced by free carbon and carbon in WC during sintering. The reaction is: MeO + C == Me + CO, MeO + 2C == MeC + decarburization reaction has occurred.

Mixing

The surface of the alloy product mixed with materials resembles the skin of a bitter melon, with uneven alloy structure. Its cross-section is different from the general dirty holes, often showing spots of varying sizes and shapes, as well as uneven surfaces. Different grades of organizational structure can be seen in the microstructure. Mixed carbide materials affect performance and are generally considered scrap, but slightly mixed materials can be inspected and treated according to the standard for cross-sectional contamination.

Causes of mixing

1.Mixing before pressing

2.The influence of certain impurity elements, such as aluminum, sulfur, silicon, phosphorus, and boron, which can cause WC grain growth during liquid-phase sintering, with phosphorus having the most significant effect.

 

Over-sintering

Over-sintering products have enlarged surface grains and coarser cross-sectional structure. In mild cases, only a larger number of shiny spots are observed, while in severe cases, the surface sometimes shows blisters or a honeycomb appearance. Over-fired products should be considered scrap.

Causes of over-sintering

1.Excessive sintering temperature – grain growth

2.Prolonged holding time – grain growth

 

Under-sintering

Under-fired alloy products have a loose structure, dark surface color, and no metallic luster. Vacuum-sintered products have a gray-white surface, larger shiny spots on the cross-section, and a noticeable water absorption phenomenon. Under-fired products should be returned to the production unit for treatment.

 

Poor pressing

This type of alloy product, due to insufficient compacting density and excessively large hole size, does not completely disappear during the sintering process. The product’s surface shows loose particles, mainly appearing at the blade edges and corners. In severe cases, fine cracks appear on the surface, and the cross-section shows triangular or strip-shaped holes. If only the surface is slightly poorly pressed, and the cross-section and metallography do not show this phenomenon, it can be released as a qualified product. If the surface is poorly pressed, and the cross-section and metallography also show this phenomenon, then this type of product should be treated as scrap.

Causes of poor pressing

Overly hard, overly coarse granular materials, uneven distribution of granular materials in the mold cavity, low compact single weight, low pressing pressure, or local low density.

 

結(jié)論

The above only analyzes several reasons for the non-conformance of carbide products. In actual production, there may be various other issues, which require us to further improve our understanding, analyze the causes, and propose specific countermeasures. After the occurrence of non-conformance, it is necessary to seriously analyze our production process, identify the causes, and make improvements. Generally, attention should be paid to details, especially the usual practices that are often taken for granted. Only by truly focusing on the details can we reduce problems and avoid quality issues. Therefore, it is said: “Details determine success or failure.”