The Influence of Undercooling DT on the Structure and Tensile Strength of Grey Cast Iron

Inoculation of cast iron has become a commonly used metallurgical process, which is carried out in a foundry in order to improve the mechanical properties of utility alloys. It consists in changing the physicochemical state of the melted alloy. This change is caused by the introduction of cast iron with a low ability to nucleate graphite, shortly before pouring a small mass of the substance—an inoculant that increases the number of active nuclei. It is also justified that the literature often connects an increase in the tensile strength UTS of the inoculated grey cast iron, with changes in the characteristics of the particles of graphite. However, in strongly hypoeutectic cast iron, in which a large number of primary austenite grains crystallize, the interdendritic distribution of graphite is usually the result. It also follows that the nature of the graphite precipitates is determined by the mutual relations between the interfacial distances in eutectic grains and the interdendritic distances in the grains of primary austenite occurring in the Fe–C alloys. The article presents the influence of the inoculant on the characteristics of the precipitation of primary austenite grains in relation to the sulphur content in grey cast iron with flake graphite. The study also showed that primary grains in grey cast iron have a great influence on mechanical properties, such as the tensile strength UTS. In this case, the key is to know the value of the degree of undercooling DT. The type of inoculant used affects the DT value. The study related the number of N primary austenite grains with the degree of undercooling DT and the tensile strength UTS with the number of primary austenite N grains.

Effect of Microstructural Bands on the Localized Corrosion of Laser Surface-melted 316L Stainless Steel

The localized corrosion of laser surface melted (LSM) 316L stainless steel is investigated by a combination of potentiodynamic anodic polarization in 0.1M HCl and microscopic investigation of the initiation and propagation of localized corrosion. The pitting potential of LSM 316L is significantly lower than the pitting potential of wrought 316L. The LSM microstructure is highly banded as a consequence of the high laser power density and high linear energy density. The bands are composed of zones of changing modes of solidification, cycling between very narrow regions of primary austenite solidification and very wide regions of primary ferrite solidification. Pits initiate in the outer edge of each band where the mode of solidification is primary austenite plane front solidification and primary austenite cellular solidification. The primary austenite regions have low chromium concentration (and possibly low molybdenum concentration), which explains their susceptibility to pitting corrosion. The ferrite is enriched in chromium, which explains the absence of pitting in the primary ferrite regions. The presence of the low chromium regions of primary austenite solidification explains the lower pitting resistance of LSM 316L relative to wrought 316L. The influence of banding on localized corrosion is applicable to other rapidly solidified processes such as additive manufacturing.

Improvement of mechanical properties of iron castings via adjusting of solidification rate

The work is devoted to improvement of mechanical properties of iron castings via adjusting of the cooling rate without introduction of alloying additives. The new technological solution is suggested; it can be easily adapted to a casting technology. This solution is based on variation of the cooling rate of iron castings within structurally sensitive solidification intervals. For this purpose, the casting mould was initially cooled after pouring, then heated and cooled again. Cooling of the mould during the period of primary austenite crystal forming led to increase of dendrite crystallization rate and was executed using compressed air. Retarding of the cooling rate during the period of eutectic transformation was provided by the mould heating via burning of exothermic carbon-containing additives introduced in a facing layer of sand-clay moulding mix. Burning reaction is accompanied by heat extraction, what steeply retarded the cooling rate within the interval of eutectic transformation. Consequent acceleration of castings cooling within the interval of eutectoid transformation was achieved via repeated air blowing through a worked reaction layer. Adjusted cooling of iron castings allowed to provide the most favourable solidification conditions, taking into account strictly individual requirements for each structurally sensitive temperature intervals. It led to increase of a volumetric part of primary austenite dendrite crystals, to decrease of eutectic transformation overcooling degree, to forming of graphite eutectics and enlargement of dispersity of pearlite component in iron. Consequently, lowering of widespread iron castings rejects takes place, among them chilling, with simultaneous improvement of metal mechanical properties. As a result, the primary and real structures were varied, what had a positive effect on mechanical properties of casting metal. It is shown that use of solidification rate adjustment led to essential increase of metal tensile strength for the experimental casting.

ON THE POSSIBILITY OF APPLYING A NEW PARAMETER OF THE PRIMARY STRUCTURE OF GRAY CAST IRON TO EVALUATE THE STRENGTH OF CASTINGS

The possibility of evaluating the strength of pearlite gray cast iron with lamellar graphite using the ratio of the area of the interdendritic eutectic phase to the perimeter of the primary austenite dendritic crystals was investigated. It is found that this parameter increases from 15,1 to 39,3 as the ultimate tensile strength of cast iron decreases from 300 to 180 MPa. The correlation coefficient for this relationship was 0.9.

STRUCTURAL CHANGES UNDER HEATING IN DENDRITIC CRYSTALS OF GREY CAST IRON

Based on approach to primary structure of grey pearlite cast iron as analogue of composite material reinforced with discrete fibers, changes occurring in dendritic crystals of primary austenite at heating to temperatures not exceeding critical point of А are analyzed.

TUNGSTEN RECOVERY FROM OXIDE DURING FLUX CORD WIRE SURFACING

Influence of introduction of tungsten powder and tungsten concentrate into surfacing flux-cored wire on structure, structural components microhardness, hardness and wear of the surfacing layer has been studied. Flux cored tungsten-containing wires of H- and E-types according to the IIW classification were manufactured for surfacing in laboratory. Powders of silicon KR-1 (GOST 2169 – 69), manganese MR-0 (GOST 6008 – 82), chromium PKhA-1M (industrial standard TU 14-1-1474 – 75), vanadium VEL-1 (industrial standard TU 48-0533 – 71), nickel PNK-1l5 (GOST 9722 – 97), aluminum PAP-1 (GOST 5494 – 95), tungsten PVT (industrial standard TU 48-19-72 – 92) and iron powder PZhV-1 (GOST 9849 – 86) were used as fillers. In some wires tungsten concentrate KSh-4 (GOST 213 – 83) produced by “AIR” mining company” JSC was used instead of tungsten powder. Gas cleaning dust of aluminum production of the following chemical composition: 21.00 – 43.27 % Al2O3; 18 – 27 % F; 8 – 13 % Na2O; 0.4 – 6.0 % K2O; 0.7 – 2.1 % CaO; 0.50 – 2.48 % SiO2; 2.1 – 2.3 % Fe2O3; 12.5 – 28.2 % Cgen; 0.03 – – 0.90 % MnO %; 0.04 – 0.90 % MgO; 0.09 – 0.46 % S; 0.10 – – 0.18 % P (by weight) was used as a carbon-containing reducing agent. Wire with diameter of 5mm manufactured at laboratory installation ASAW 1250 tractor was used for surfacing. Surfacing modes were: Is = 400 – 450 A; Ud = 32 ÷ 36 V; Vs = 24 ÷ 30 m/h. Surfacing was performed under a layer of AN-26S flux and flux made of silicomanganese slag; number of deposited layers – 5. Chemical composition of deposited metal was determined, metallographic analysis of deposited layer was carried out: size of the former austenite grain, size of martensite needles, degree of contamination by nonmetallic inclusions were stated and wear tests were carried out, hardness and microhardness were measured. The possibility in principal of using tungsten concentrate instead of tungsten powder in studied flux cored wires is shown, degree of tungsten extraction was calculated. For H-type fluxcored wire, introduction of tungsten concentrate instead of tungsten powder into the charge of wire does not increase contamination of deposited layers with nonmetallic inclusions and reduces size of the primary austenite grain. Use of tungsten concentrate in E-type flux-cored wire manufacturing helps to reduce size of the primary austenite grain and size of martensite needles, increasing microhardness of martensite in structure of deposited layer. Introduction of tungsten concentrate instead of tungsten powder into the composition of the charge of H-type wire provides a significant increase in hardness and wear resistance of deposited layer.