D.I. Martínez

Effect of Preweld Heat Treatment on the Microstructure of Heat-Affected Zone (HAZ) and Weldability of Inconel 939 Superalloy

The effect of two preweld heat treatments on the final microstructure of a heat-affected zone (HAZ) and its effect on the weldability of the superalloy Inconel 939 have been analyzed. The HAZ cracking related to the mechanical driving force criteria during the cooling cycle was directly influenced by the strain state at this zone. Heat treatments result in the formation of (1) irregular γ′ precipitates (more negative misfits) which showed higher strain (high density of interfacial dislocations) and (2) spherical precipitates (near-zero misfits) which as a result of their structure improve weldability by reducing weld HAZ cracking considerably.

Study of the Tempcore Process for the Production of High Resistance Reinforcing Rods

The process known as Tempcore is used to produce high resistance rods by the formation of a surface layer of quenched and tempered martensite that surrounds a core made of ferrite and pearlite. Such a mixed structure is result of processing hot rolled rods through waters headers that reduce the temperature at the surface below that for the transformation into martensite. This structure is tempered by the heat flowing from the core of the rod, which transforms into ferrite and pearlite while the rod is in the cooling beds. Such processing produces a significant increase in yield and ultimate tensile strength, while maintaining adequate ductility. The economic advantages of this process are huge in comparison with those that require alloying elements or further metal working to improve mechanical properties. A series of experimental trials were carried out in a pilot plant in which parameters such as reheating temperature, water flow and processing time were varied to study their effect on the mechanical properties of carbon steel rods and on the structures formed in the bars. The study is being complemented by the thermal modelling by the finite element method.

Wear in pneumatic transport under hot reducing atmospheres

Abstract Samples of steel pipes and pipe segments were subject to multiple impacts by direct reduced iron pellets, exposed to hydrogen and carbon monoxide gas mixtures, at temperatures ranging from 300 to 600°C. Material loss and microscopic observations were used to evaluate wear. The results are useful for material selection and preventative maintenance planning. Erosion wear was the important mechanism. Comparison between laboratory test and pilot plant was made and their analysis used to provide predictions of wear rates.

Aging Effect on the Microstructure of the Superalloy Inconel 939

In this work were evaluated the microestructural characteristics by Optical Microscopy (OM) and Scanning Electron Microscopy (SEM) of the intermetallic ’ (liquation, coarsening and decomposition) in the Inconel 939 alloy after 40000 hours and 850-900°C aging operation conditions. The alloy was vacuum conventional cast. The results show that the liquation phenomena take place in eutectics ’ which are present mainly in the core of the dendritic arms and in the coarse films of carbides along the grain boundaries (GB), the ’ particles change their original morphology of ordered cuboids of 320nm to disordered and coarse cuboids of 1.2 m, carbides show a morphology change from the original dispersed particles into a coarse continuous films and particles Chinese script type, this affects adversely the mechanical properties such as creep. The results of this evaluation allow to determine the main microestructural damage mechanisms which experiment some components such as blades at high temperatures in industrial conditions.

Characterization of the Microstructural Degradation of Platinum Modified Aluminide Coating

This work presents the degradation mechanism of the platinum modified aluminide diffusion coating of the GTD 111 SC Ni-base superalloy turbine blades after 16000 h of exposition at different thermal cycles (critical heating temperatures reported ~1000°C and 1120°C). The initial coating condition and the evolution of degradation were characterized applying conventional microscopy and backscatter scanning electron microscopy. The initial microstructure condition consisted of a two phase coating (intermetallics PtAl2 dispersed in a matrix β-(Ni,Pt)Al). The major microstructure degradation was associated with: intermediate and interdiffusion zones growing, partial transformation of β-(Ni,Pt)Al to γ´-Ni3Al, and the dissolution of the intermetallic PtAl2 resulting in a more brittle single phase β-(Ni,Pt)Al coating. The degradation facilitates spallation and crack initiation, resulting in the loss of the coating and by consequence the blade failure.

Light-beam soldering of a lead free alloy

Light-beam joints made from a Sn-Ag-Cu lead free solder alloy were studied. Joints were prepared with the five combinations of variables that resulted with the least amount of defects; these joints were tested in tension to evaluate their load bearing capacity. Solder pads made following these conditions were aged to study the growth of the intermetallic compound layer and its effect on the resistance of the joints. It was found that aging engrosses this layer and that the bearing capacity increased until the layer reaches 1.5 µm; further thickening reduces the resistance of the joints.

Erosion in Hard Coatings in Pneumatic Conveying of Direct Reduced Iron Pellets

Five samples of hard coatings (high chromium–high carbon) were separately tested for hot erosion resistance by multiple impacts of direct reduced iron (DRI) pellets in a hot erosion testing device. The volumetric wear and microscopic observations were used to evaluate the erosion mechanisms at different angles of impact. Microstructural parameters such as the partial area of the hard phase particles (carbides), the distance between the particles, and the mean free path were measured and correlated with the volume lost. The results of this work show a correlation between microstructural parameters, mostly the mean free path, with erosion resistance. Hot corrosion was inhibited by the use of a hydrogen inert atmosphere for the erosion testing. However, the hard coating samples were tested for hot corrosion in a reactor at 700°C. Metal dusting was present in stainless steel, but this kind of corrosion was not detected in the hard coatings. The results obtained here will help to evaluate the main mechanisms of pipe damage used in the high-temperature pneumatic transportation of DRI under industrial conditions.