1. Effect of temperature on mechanical properties of steel P22 subjected to a constant tensile stress NGUYỄN THU HIỀN, NGUYỄN CAO SƠN, PHÙNG THỊ TỐ HẰNG, BÙI ANH HÒA* Viện Khoa học và kỹ thuật Vật liệu, Trường Đại học Bách khoa Hà Nội This paper presents the effect of temperature and stress on the mechanical properties of steel pipe in the ther- moelectric power plant. Steel samples were cut from the raw pipe P22 of a Vietnamese power plant, heated at tem- peratures of 300, 400 and 500 ^oC for 72 hours under a constant tensile stress. The results showed a clear influ- ence of heating temperature on the mechanical properties of the steel: the tensile strength and the yield strength decreased with increasing temperature. Microstructure of the steel, heated at various temperatures, was found to be invisibly changed; carbides were distributed both inside the grains and the grain boundaries. Carbides were con- firmed to contain some elements such as Cr, Mo and Si whose contents did not change much with temperature. Meanwhile, the precipitation of carbide on the grain boundaries was found by SEM observation when the temper- ature increased from 300 to 500 ^oC. This is considered as one of reasons for the decrease of the steel strength when heating the steel P22 under the contant stress. # strength, microstructure, steel grade P22, carbide Page: 2 - 6 RESEARCH Get article

2. Effect of heat treatment in three-phase region on ferrite fraction of an austempered ductile iron with dual matrix structure

HOÀNG ANH TUẤN Viện Công nghệ, Tổng cục Công nghiệp quốc phòng, Bộ Quốc phòng, Số 3 Cầu Vồng, Bắc Từ Liêm, Hà Nội NGUYỄN HỮU DŨNG, NGUYỄN HỒNG HẢI Viện Khoa học và kỹ thuật Vật liệu, trường Đại học Bách khoa Hà Nội, Số 1 Đại Cồ Việt, Hà Nội Austempered Ductile Iron (ADI) with dual matrix structure has exhibited the best combination of high strength and ductility, good wear resistance and good machinability. Microstructure of this iron consists of proeutectic fer- rite and ausferrite. Ausferrite is a mixture of acicular ferrite and high carbon austenite. For this purpose, ductile iron specimens were heated to fully austenitizing temperature in the range of 900 to 930 ^oC for 120 minutes, followed by annealing in three-phase region for 90 minutes and then austempered at temperatures (300-500) ^oC for (30-120) minutes. This treatment was aimed to create a dual structure constitued of proeutectoid ferrite and ausferrite. Mechanical properties of materials depend on ferrite volum fraction in the structure, in other words, depend on the heat treatment parameters in the three-phase region. This paper presents the influence of austenitization temper- atures, holding temperatures and times in the three-phase region on ferrite and ausferrite volum fration in dual matrix ADI. # Austempered Ductile Iron, dual matrix ADI, proeutectoid ferrite Page: 7 - 13 RESEARCH Get article

3. Effects of heat treatment on properties of Cu₃NiSi alloy for welding electrode

HOÀNG VĂN QUÂN, NGUYỄN MINH ĐẠT, LÊ VIỆT DŨNG Viện Khoa học và Công nghệ Mỏ - Luyện kim, 79 An Trạch, Quốc Tử giám, Đống Đa, Hà Nội DƯƠNG NGỌC BÌNH Viện Khoa học và Kĩ thuật Vật liệu, Trường Đại học Bách Khoa Hà Nội Welding electrode, a very important part of industrial welding machines, was often made of Cu-Cr or Cu-Be alloys. However, alternative alloys such as the Cu₃NiSi are being studied as a replacement in welding electrode application because of the toxicity of Cu-Be alloys. In this study, Cu₃NiSi alloy with approx. 3,0 %Ni and 1,0 %Si is fabricated using melting process. The casted alloy was then annealed at 900 ^oC for 2 hours, forged and re- annealed at 600 ^oC for 2 hours. The alloy was then quenched at (825 ÷900) ^oC in oil (or water) and aged at (400÷500) ^oC. Results show that Cu3NiSi alloy has the highest mechanical properties when quenched at 900 ^oC in oil and aged at 450 ^oC for 4 hours. Tensile strength of the alloy reaches 517 MPa and hardness is of 244 HB. The conductivity of the alloy is 39.1 %IACS. The manufactured welding electrodes were used for chain welding lines of Mao Khe Mechanical Joint Stock Company - Quang Ninh. # Welding electrode, Cu3NiSi copper alloy, heat treatment Page: 14 - 18 RESEARCH Get article