Analysis of Cracking Causes and Thermal Brittleness Mechanism of CW617N Brass Ball Valve

Brass has the advantages of high strength, corrosion resistance, low temperature resistance, good processability and conductivity, and has been widely used in industries such as electric power, communications, transportation, chemical industry and container manufacturing. Single-phase copper (α-phase) is not suitable for hot processing. Therefore, dual-phase brass alloys (α+β) with better thermal processing properties are often used to produce hot forged products, such as valves, faucets, and pipe connectors. However, in the process of hot forging, brass alloys need to withstand large deformations at high temperatures. In addition, the structure of hot forged products is complex, which can easily produce defects such as cold barriers, peeling, folding, and coarse grains.

After hot forging, a certain brand of dual-phase CW617N brass ball valve cracked due to thermal embrittlement or leaked in the tightness test. In response to this phenomenon, the author inspected and analyzed the crack morphology, propagation path, and cause of formation, and studied the hot brittle mechanism of dual-phase brass alloys in order to take measures to avoid similar failures from happening again.

The SPECTRO LAB LAVM10 direct reading spectrometer was used to analyze the chemical composition of the cracked valve body. The results are shown in Table 1. It can be seen that the chemical composition of the brass alloy used in the hot forging valve meets the requirements of the EN 12164:2011 Copper and Copper Alloys-Rod for Free Machining Purposes standard.

After leak detection, it was found that the crack was located in the valve body and expanded in the axial direction, as shown in Figure 1a). Intercept the cracked part of the valve body and observe the crack surface morphology. It can be seen from Fig. 1b) and Fig. 1c) that there are no obvious plastic deformation, scratches and bump marks near the crack; the surface of the crack is flat and expands in a zigzag manner. Therefore, it is preliminarily judged that the valve body is brittle and cracked.

It can be seen that the crack-free area and the microstructure near the crack are composed of α phase (white) and β phase (black); the grain size is basically the same, and there is no significant difference, all are equiaxed crystals; no abnormal microstructure is seen in the crack area . It can be seen from Figure 2b) and Figure 2c) that both the main crack and the secondary crack propagate along the grain boundary, that is, the fracture form is intergranular cracking.

The crack is opened along the crack propagation direction, and the micro morphology of the fracture is observed by scanning electron microscope (SEM). It can be seen from Figure 3a) and Figure 3b) that the fracture surface is relatively flat without plastic deformation; the surface of the fracture surface is blocky or rock sugar, with a certain amount of secondary cracks, and no plastic fracture morphology such as dimples. It shows that the failure mode of the CW617N brass ball valve is typical intergranular brittle cracking.

In order to find out the cause of the crack along the crystal, the surface morphology of the fracture was further magnified and observed, and some substances distributed along the grain boundary were seen. They were white and bright in the SEM backscatter mode, as shown in Figure 3c). The energy spectrum (EDS) analysis of the white and bright color material shows that its chemical composition is mainly lead element, indicating that the phenomenon of lead segregation has occurred on the grain boundary.

Fracture analysis shows that the cracking form of CW617N brass ball valve is brittle fracture along the crystal. Generally speaking, the bonding force of the grain boundary is higher than the bonding force within the grain. Only when the grain boundary is weakened, the crack will propagate along the grain boundary and cause brittle fracture. The main reasons for the weakening of material grain boundaries include: inclusions on the grain boundaries or continuous brittle precipitation phase; impurity elements phosphorus, sulfur, arsenic, antimony, tin, bismuth, lead, etc. segregate at the grain boundaries; environmental media factors Causes corrosion, high temperature creep, etc.

During the service process of brass products, due to the effect of stress corrosion, brittle cracks along the crystal often occur. However, the failed CW617N brass ball valve in this study was not in service, and there were no inter-crystalline precipitated phases or inclusions in the microstructure near the cracks, which can eliminate the influence of environmental corrosion factors and inter-crystalline precipitated phases or inclusions. The EDS analysis results of the residual substances in the fracture surface and microstructure show that there is segregation of lead at the grain boundary, which is the main reason for the weakening of the grain boundary of the CW617N brass ball valve. Coupled with the effect of the tensile stress generated during the hot forging process or the residual stress during the cooling process, it is very easy to induce the initiation and propagation of intergranular cracks, which will eventually lead to fracture failure.

CW617N brass ball valve cracking along the crystal is mainly due to hot brittleness during hot forging. The hot embrittlement phenomenon of brass means that during the hot working process, the low melting point eutectic melts first, which leads to the weakening or embrittlement of the brass grain boundary, and brittle fracture occurs under the action of external stress. The mass fraction of lead in the brass raw material used in the valve is about 2%, and the solubility of lead in brass is less than 0.3%, and most of it exists in the brass in the form of free lead particles. Lead and copper easily form a low melting point eutectic structure, and the eutectic temperature is only 326°C. The hot forging process is high-temperature extrusion molding. Under the action of tensile stress during the extrusion and cooling process, the material is prone to brittle fracture along the crystal, that is, hot brittleness. The hot brittleness phenomenon in the hot forging process of brass is related to the chemical element segregation of the raw material, the unreasonable high temperature residence time, the extrusion speed and the cooling rate during the hot forging process.

The failure mode of the CW617N brass ball valve is intergranular cracking failure caused by thermal embrittlement. During the hot forging process, the unreasonable hot forging process causes the lead element to gather at the grain boundary to form a lead-rich low-melting eutectic phase, and under the action of tensile stress, the intergranular microcracks are formed. The cracks originate from the stress concentration on the surface of the valve body, and expand along the axial direction, eventually leading to fracture failure.

In order to prevent the hot brittleness of hot-forged brass, attention should be paid to strictly control the content of impurity elements in raw materials to reduce element segregation; formulate a reasonable hot forging process to prevent the material from staying in the brittle temperature range of 300~400℃ for a long time; reduce the extrusion speed And the cooling rate, so as not to produce higher internal stress.

Selected from: "Physical and Chemical Inspection - Physics Volume" Vol.55 2019.8