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A failure investigation has been conducted on high carbon microalloyed steel used in engine fracture splitting con-rods. The fracture occurred when the as-rolled bars are cut down. The study involved the in-depth examination of the chemical composition, microstructures, mechanical properties and fracture morphologies of specimens from failed bars. Results indicated that the chemical composition of the as-rolled material, microstructure, and mechanical properties are within the range of the technical specification. Microstructure is ferrite-pearlite with dominance of pearlite due to the relatively high carbon content. SEM of fracture surfaces of failed as-rolled bars illustrates that the fractography is cleavage steps and river line pattern. It indicates that failure mode is brittle and the mechanism of fracture is cleavage. Microfracturography shows sulphur inclusions and oxides are the fracture initiation. The continuous network proeutectoid ferrite at the edge of fracture bar will prodigiously improve the brittleness and quickly bring stress concentration under dynamic loading conditions. Furthermore, coarseness and narrowband shape ferrite will induce to decrease toughness.

The interest in the effect of firefighting foams on the environment has increased. Biodegradation is an important influencing factor when considering the environmental effect of firefighting foams. In this paper, an evaluation system based on CO2 evolution method was established to evaluate the biodegradation of Class A foams, and the biodegradation of three kinds of Class A foams were compared. The results show that both aniline and sodium benzoate can be employed as reference compounds using CO2 evolution method. Three kinds of Class A foams had biodegradation rates of 88.64%, 85.27% and 88.62% respectively after 28 days, which means they are easily biodegradable. Data from different test methods could not be compared directly.

Firefighting foam is highly effective in suppressing and extinguishing liquid fuel fires. As the concern of its environmental impacts growing, it is necessary to evaluate its biodegradation in the environment. This article briefly introduces the concept, classification, and the main component of firefighting foam. Also the principle, general process, basis mechanism of the biodegradation of firefighting foam are discussed. There is no standard test methods focusing on the biodegradation of firefighting foam, also relative research and reports are less. Limited research data abroad shows that the biodegradability of some foams are very low, and protein based foams may not easier to biodegrade than synthetic based foams.

The microstructure and precipitates of the V-N microalloyed steels whose carbon content were respectively 0.27% and 0.35%, were investigated by scanning electron microscope (SEM) and transmission electron microscope (TEM). It is shown that the microstructures of the test steels are all composed of ferrite and pearlite, the area percent of ferrite decreases from 49% to 40% and the grain size also descends from 8.96μm to 8.61μm with the increase of carbon content. TEM results show that the precipitates in two kinds of steels all include a large amount of 10~20nm dispersion distribution irregular flake VC, the part of fibrous VC that grows toward to intragranular ferrite along the grain boundary in the local area, and only a small amount of spherical VN or V (C, N). When the carbon content increases from 0.27% to 0.35%, the number of the spherical VN or V(C,N) increases obviously and the size of it varies from 20~100nm to 45~105nm, while the number of flake VC and fibrous VC decreases significantly and the length of fibrous VC shortens from several micrometers to nanometer size. Experimental results indicate that most of the spherical VN or V (C, N) firstly appear in austenite, then the flake VN and fibrous VN with precipitation strength effect emerge in ferrite during the following γ→α transformation and cooling process. The increase of carbon content can lead to the increase of driving force that VN or V (C, N) firstly appear in austenite, which results in the significant increase of ferrite nucleation rate and the refinement of microstructure.

The microstructure and precipitate of the two kinds of medium-carbon vanadium microalloyed steels whose nitrogen contents were 0.0035% and 0.012% respectively, were studied by image analysis and transmission electron microscope (TEM). The results show there are the large amount of 10~20nm dispersion distribution irregular flake VC precipitates within the ferrite, the part of clustered fibrous VC precipitates with the diameter of 4~13nm that grow toward to ferrite intracrystalline along the grain boundary with some angle in the local area, and only a very small amount of 20 ~ 50nm spherical particles V (C, N) in the low-nitrogen steel. However, in the high-nitrogen steel, the precipitates are divided into two stages: the first stage is the part of 30~80nm spherical particles V(C, N) which precipitation in austenite, the second stage is flakiness VC which precipitation in ferrite during the following γ → α phase transformation and cooling process. Compared with the low-nitrogen steel, the number of precipitates in decreased significantly and the size increased obviously in the high-nitrogen steel. The substantial increase of nitrogen content leads to the rapid increase of driving force that V (C, N) precipitation in austenite. A lot of V (C, N) that precipitation before phase transformation results in the significant increase of ferrite nucleation rate, which leads to the microstructure of high-nitrogen steel fined obviously.

The effect of cooling rates after finish rolling on microstructure and properties of high strength medium carbon microalloyed steels used for fracture splitting con-rods was investigated using the methods of optical microscopy, SEM and TEM, etc. The results show that high cooling rate after finish rolling can increase the percentage of pearlite, reduce the grain size of ferrite and lamellar spacing of pearlite. The precipitations in the steel are composite phases of (V,Ti)(C,N), etc. Their granularity are about 30-170nm, and they will decrease with the speeding up cooling. The yield strength (YS) and ultimate tensile strength (UTS) are improved with increasing the cooling rate. The impact fracture surface shows distinct brittle fracture character.

The fracture splitting method is being increasingly used for manufacturing connecting rods that are made of the powder forging material, high-carbon steel, etc., which are comparatively easy to fracture, in order to improve the accuracy and productivity. In this paper, a microalloyed high carbon steel used for fracture splitting connecting rod is developed by the process “EAF-EBT-LF-CC-CR”. The steel microstructure, nonmetallic inclusions, precipitated phases, mechanical property and fracture surfaces are investigated by microscope, SEM and TEM. The results show that the steel is constituted of pearlite and a small mount of ferrite, the nonmetallic inclusions in the steel include MnS, Al-rich (taken to be Al2O3) and Al-Ca-rich (taken to be CaO-Al2O3) particles. The precipitations mainly are VC, VN, (Mn, Cu)S, etc. these precipitations are beneficial to fine the pearlite group, short lamellar spacing of pearlite and improve the strengthening of the microalloyed high carbon steel. The tensile tests indicate that the microalloyed high carbon steel has high yield strength, ultimate tensile strength (UTS), but low elongation and contraction of area. It is required for heavier loads, higher performance of automobile engine and less distortion on fracture splitting. The fracture surface of connecting rod shows distinct brittle fracture character and it is beneficial to improve fracture splitting performance.