Laser surface modification of carburized and borocarburized 15CrNi6 steel

The paper presents the results of laser heat treatment (LHT) of carburized and borocarburized 15CrNi6 low-carbon steel. Laser tracks were arranged by CO₂ laser beam as multiple tracks formed in the shape of a helical line. The microstructure and properties of these diffusion layers were compared with those obtained after through-hardening. The microstructure after carburizing and LHT consists of adjacent characteristic zones: re-melted zone (coarse-grained martensite), carburized layer with heat affected zone (fine acicular martensite), carburized layer without heat treatment and the substrate (ferrite and pearlite). The highest measured microhardness (about 820 HV) was observed in re-melted and heat affected zones. The increase of distance from the surface was accompanied by a gradual decrease of microhardness up to 400 HV beneath the HAZ and up to 250 HV in the core of steel. The carburized layer after LHT exhibited a higher resistance to frictional wear compared to a carburized layer after through-hardening. The microstructure after borocarburizing and LHT consists of the following characteristic zones: iron borides of laser-modified morphology (FeB and Fe₂B), carburized layer with heat affected zone (martensite and alloyed cementite), carburized layer without heat treatment and the substrate (ferrite and pearlite). The highest microhardness was obtained in the iron boride zone. The microhardness of FeB boride extended up to 2200 HV and for the Fe₂B boride up to about 1300–1600 HV. With increased distance from the surface, the microhardness gradually decreases to 800 HV in HAZ, 400–450 HV in the carburized layer without heat treatment and to 250 HV in low-carbon substrate. The iron borides after LHT assume a globular shape, which leads to a lower texture and porosity of the borided layers. The increased resistance to friction wear of the borocarburized layers is certified in comparison with the borided layer after conventional heat treatment (through-hardening).     

Laser surface modification of 316 L stainless steel with bioactive hydroxyapatite

Laser-engineered net shaping (LENS?), a commercial additive manufacturing process, was used to modify the surfaces of 316 L stainless steel with bioactive hydroxyapatite (HAP). The modified surfaces were characterized in terms of their microstructure, hardness and apatite forming ability. The results showed that with increase in laser energy input from 32 J/mm² to 59 J/mm² the thickness of the modified surface increased from 222 ± 12 μm to 355 ± 6 μm, while the average surface hardness decreased marginally from 403 ± 18 HV_0.3 to 372 ± 8 HV_0.3. Microstructural studies showed that the modified surface consisted of austenite dendrites with HAP and some reaction products primarily occurring in the inter-dendritic regions. Finally, the surface-modified 316 L samples immersed in simulated body fluids showed significantly higher apatite precipitation compared to unmodified 316 L samples.     

Laser surface melting of a complex high alloy steel

The effect of laser surface melting (LSM) on the microscopic morphology of a complex high alloy steel has been examined in single track experiments. Different combinations of power (1–4 kW) and scan speed (500–1000 mm min⁻¹) were selected in order to obtain a range of depths of melt penetration. An increase in the depth of melt penetration has correlated with rise in the estimated surface temperature induced by laser melting. Within the strongly refined structure of the laser melted alloy, the interdendritic spacing, λ varied from 0.29 μm to 0.97 μm with increase in the depth of melt penetration. The microhardness in the laser surface melted region has been shown to decrease linearly with λ^−0.5.     

Laser surface modification of metals and alloys

In order to understand the properties of solids, the materials scientist frequently needs to characterize the microstructure of a material on as fine a scale as possible. Characterization here is used in its most general sense, i.e. to describe as completely as possible the morphology, crystallography and elemental composition of the material under investigation. Although instruments have been developed over the years that can provide this information, they generally have one common limitation, namely spatial resolution. Analytical electron microscopy (AEM) is the term which is now applied to a new group of analytical techniques used in conjunction with a transmission electron microscope and/or a scanning-transmission electron microscope that can provide high spatial resolution (about 20 nm) analyses from microvolumes of material in thin electron-transparent specimens. The basic principles of microanalysis using AEM are discussed, together with examples of application in both metallurgical and ceramic studies.     

Plasma transferred arc surface modification of a low carbon steel

The potential of Plasma Transferred Arc (PTA) hardfacing goes beyond the surface welding of superalloys. This work evaluated low carbon steel surface modification by PTA deposition of fine WCoC carbides, and mixtures of Fe powders and 5–35 wt% carbides. Characterization included visual inspection, optical and scanning electron microscopy, X-ray diffraction and microhardness profiles. PTA processing allowed for the dissolution of carbides confirmed by X-ray diffraction, leading to homogeneous microstructures. Microstructures varied from a Widmanstätten morphology to a typical dendritic solidification structure upon the WCoC content. Surface soundness depended on powder preparation and composition. Sound surfaces exhibiting hardness up to 700 Hv were obtained for the 35 wt% WCoC powder mixture.     

Characterization of boriding of 0.3% C, 0.02% P plain carbon steel

In the present study, some mechanical properties of borides formed on the impurity-controlled plain carbon steel essentially containing 0.3% C, 0.02% P in weight was investigated. The formation of borides on the concerned steel was performed in a bath having borax, boric acid and ferro-silicon at 940 °C for various length of time between 1/2 and 10 h. Optical and scanning electron microscope (SEM) studies revealed that borides formed on the surface of steel substrate have columnar morphology. The hardness of borides is higher than 1500 DPN. The presence of borides (FeB and Fe₂B) was confirmed by X-ray diffraction (XRD) analysis.     

Composition and microstructure of arc-sprayed 13% Cr steel coatings

A series of 13% Cr steel wear-resistant coatings have been produced by arc spraying using a range of spraying conditions. The resultant coatings have been examined with optical and scanning microscopes. It has been established that the range of particle sizes varies across the spray beam particularly if the gun-substrate distance is short. At the centre of the beam where the particles are larger, it has been shown that the mean particle are S_A is related to the spraying distance d by a relationship of the form.

where n is an exponent. It is suggested that the gun emits large particles which disintegrate in flight. The use of a small aperture in an arc-restricting air cap produced a general reduction in particle size across the spray beam at all gun-substrate distances.X-ray analysis demonstrated that the combined iron and chromium contents of the deposits were reduced when comparison was made with the composition of the original wire. The longer spraying distances (i.e.d > 50 mm) and use of the finer air caps also reduced the metallic content of the coating. Spot analysis on spray particles revealed a central metallic region with a small residual oxygen content and an outer region of varying thickness (depending on spraying conditions) which contains a significant fraction of oxide.     

Laser surface modification of UNS S31603 stainless steel. Part II: cavitation erosion characteristics

Austenitic stainless steel UNS S31603 was laser surface alloyed with various elements (Co, Ni, Mn, C, Cr, Mo, Si) and alloys/compounds (AlSiFe, Si₃N₄ and NiCrSiB) as presented in Part I together with the microstructures and the corrosion characteristics of the alloyed specimens. In Part II, the cavitation erosion characteristics of the alloyed specimens in 3.5% NaCl solution at 23°C were studied by means of a 20 kHz ultrasonic vibrator at a peak-to-peak amplitude of 30 μm. The hardness profile and the compositional profile of the alloyed layers were investigated by a Vickers hardness tester and by EDX respectively. The cavitation erosion resistance of specimens alloyed with AlSiFe, C and NiCrSiB were highest, reaching 11.1, 10.5 and 7.9 times that of the substrate respectively. The damage mode was identified to be ductile fracture for specimens containing austenite as the major phase, and brittle fracture when the major phase was ferrite or intermetallic. Cavitation erosion was initiated at the phase boundaries where there was an abrupt change in mechanical properties (e.g. hardness) and then propagated into the weaker phase. It was also noted that large improvement in cavitation erosion resistance and corrosion resistance could not be simultaneously achieved in the present study.     

Laser surface modification of polymers to improve biocompatibility

The effect on neutrophil chemokinesis and fibroblast adhesion of changing surface topography was examined using two polymeric substrates; polycarbonate and polyetherimide, modified by laser treatment to produce pillars of varying dimensions on the surfaces of these materials. The dimensions for the pillars were 7,25 or 50 m square, 0.5, 1.5 or 2.5 m deep. Human neutrophils were isolated, by centrifugation, on ficoll from heparinized whole blood obtained from healthy volunteers. Isolated neutrophils were exposed to the surfaces for 20 min and tracked using image processing and analysis techniques. The mean speed for each cell on each surface was calculated and this data statistically analysed using multivariate analysis of variance to determine any significant effect on speed of movement due to the surface topography. Compared to the potent stimulator FMLP all surfaces did not stimulate significant cell movement, but within the groups some surfaces had more effect on cell movement than others, and were stimulating cells to move faster than on the same untextured surface. Surface topography can stimulate neutrophils to move at different speeds across a surface. L929 fibroblasts were incubated on the surfaces for 48 h and then examined using scanning electron microscopy to study fibroblast position and adhesion with respect to the pillars. No pattern of orientation with respect to the pillars were observed and fibroblasts spread and elongated whether in contact with the pillars or on a smooth area of the material.     

Laser surface hardening of thin steel slabs

The transformation hardening of steels by surface heating by a c.w. laser beam has been studied. We examined the surface treatment of thin steel slabs by a suitable mathematical model of the thermal transient induced by laser beam heating. The laser parameters for surface hardening of such samples and the resulting microstructures are discussed. Hardening depths calculated from the mathematical model fit well with experimental results.     

Hot deformation behavior of an 8% Cr cold roller steel

An 8% Cr cold roller steel was compressed in the temperature range 900–1200 °C and strain rate range 0.01–10 s⁻¹. The mechanical behavior has been characterized using stress–strain curve analysis, kinetic analysis, processing maps, etc. Metallographic investigation was performed to evaluate the microstructure evolution and the mechanism of flow instability. It was found that the work hardening rate and flow stress decreased with increasing deformation temperature and decreasing strain rate in 8% Cr steel; the efficiency of power dissipation decreased with increasing Z value; flow instability was observed at higher Z-value conditions and manifested as flow localization near the grain boundary. The hot deformation equation and the dependences of critical stress for dynamic recrystallization and dynamic recrystallization grain size on Z value were obtained. The suggested processing window is in the temperature range 1050–1200 °C and strain rate range 0.1–1 s⁻¹ in the hot processing of 8% Cr steel.     

离子束辅助沉积技术(IBAD)在冷作模具表面改性上的应用

IBAD是离子注入及沉积技术发展起来的材料表面改性方面的综合技术,工模具经IBAD处理后现场使用寿命可提高2～4倍。材料经IBAD处理后改性层的硬度明显提高、摩擦系数减小、耐磨性增强;改性层是由(Ti2N+TiN)、(α-Fe+TiN)、(α-Fe(N)+Cr7C3)三层次组成,与基体间附着性好、晶粒细化呈纳米相。     

不锈钢表面的化学改性与防冻性能研究

以硅氧烷为疏水剂,通过液相、气相沉积方法在不锈钢表面成膜,构建稳定的疏水性表面。通过静态接触角及滚动角对其进行表征,对改性后的不锈钢进行相同条件的冰冻,比较其融冰时间,研究其防冻性。结果表明:十六烷基三甲氧基硅烷液相成膜的疏水性膜性能最好,接触角为118o,滚动角13o;十八烷基三甲氧基硅烷气相成膜的不锈钢片防冻能力最佳,融冰时间最短60s,不锈钢表面沉积疏水膜能够提高其防冻性能。     

Laser surface modification of Ti-6Al-4V alloy

Hardening of Ti-6Al-4V alloy with laser surface melting (LSM) and laser surface alloying (LSA) techniques was attempted. Both LSM and LSA were carried out in a nitrogeneous atmosphere. Niobium, molybdenum and zirconium were used as alloying elements in the LSA. A hardness increase was observed for both LSM and LSA. Maximum hardness was obtained for LSM and zirconium alloy addition. In LSM, hardness increased almost three-fold in comparison to the substrate, which has a Vickers hardness of 350, by the formation of TiN in the region of 100 m melt depth. Hardness then decreased slowly and reached a minimum of 580 VHN at the maximum melt depth of 750 m. However, hardness for the zirconium alloy addition was uniform throughout the melted zone. Ageing treatments were performed for all specimens at 450C and different ageing times. Hardness measurements and X-ray diffraction were utilized to delineate the features associated with the hardening of the melted zone.     

Laser surface melting of electro-metallurgic WC/steel composites

Laser surface melting was used to treat electro-metallurgic WC/steel composites. The microstructure and properties of the melted zone were investigated. It was found that a homogenous and fine microstructure was formed in the melted zone. A significant phase precipitated was the herringbone eutectic carbide of Fe₃W₃C. The volume fraction of the eutectics was related to the thermal absorption increasing with the decrease of scanning rate. Preheating was beneficial to form the orientation dendrite structure and to increase the volume fraction of the eutectics. The laser melted surface possessed better microhardness and wear resistance compared to the substrate. The volume fraction of the eutectics played an important role on the wear resistance which increased with the increase of the volume fraction of the eutectics. The wear resistance was mainly depended on the content of the primary WC, as increased with the increase of the content of WC. The improvement of interface bonding, fine eutectic precipitation and structure refinement etc. were regarded as the micro-mechanism of enhancing the surface properties.     

Phase transformations in a super ferritic stainless steel containing 28% Cr after nitrogen ion implantation

A super ferritic stainless steel with 28.12% Cr; 3.91% Ni; 2.44% Mo; 0.22% Mn; 0.35% Si; 0.01% C; 0.01% N was implanted with nitrogen ions at 60 and 100 keV with different implantation doses. The nitrogen distribution and the microstructure of the implanted layers were analysed by means of nuclear reaction analysis (NRA), Mössbauer spectroscopy (CEMS) and transmission electron microscopy (TEM). A nitrogen saturated austenite was formed at a lower implantation dose, this austenite led to the precipitation of -Fe₁₆N₂ nitrides and chromium nitrides by increasing the implantation dose and the implantation energy.     

Creep properties of 12% Cr and improved 9% Cr weldments

Martensitic Cr-alloyed high-temperature materials offer interesting opportunities for design and construction of advanced power plants. An extensive research programme has been carried out at the Research Centre of the Belgian Welding Institute and Laborelec on martensitic 12% Cr steel for gaining a better understanding of the failure mode and the deformation mechanism of welded joints under uniaxial and multiaxial loads. A large number of pipe girth welds were realized by three Belgian manufacturers (Cockerill Mechanical Industries, Fabricom and Mannesmann-Carnoy). Different filler metals were used and the influence of the welding regime (austenitic and martensitic), the post-weld heat treatment (PWHT) (single or double) and the base metal wall thickness on the high-temperature properties of the different weldments was evaluated. It was found that the creep properties of a 12% Cr weldment are not influenced by the welding regime and the base metal wall thickness. As would be expected, the creep strengths of the original 12% Cr base metal as well as the temperature of the PWHT have some effect. The existence of a typical failure in the intercritical zone (type IV region) is demonstrated and explained. The consequences for the design of welded 12% Cr components are indicated. More recently the research was extended towards improved 9% Cr steel (T91). A rather small preliminary programme for the orientation of further research showed a similar failure location as for 12% Cr steel, although the observed loss in strength of the weldment compared to the base metal tended to be considerably lower. The so-called ‘half-tempering’ treatment was tried out and the effect on the creep strength of the weldment is shown. A more fundamental national research programme on P91 steel has been established and is actually running.     

Anomalous steam oxidation behavior of a creep resistant martensitic 9 wt. % Cr steel

The efficiency of thermal power plants is currently limited by the long-term creep strength and the steam oxidation resistance of the commercially available ferritic/martensitic steel grades. Higher operating pressures and temperatures are essential to increase efficiency but impose important requirements on the materials, from both the mechanical and chemical stability perspective. It has been shown that in general, a Cr wt. % higher than 9 is required for acceptable oxidation rates at 650 °C, but on the other hand such high Cr content is detrimental to the creep strength. Surprisingly, preliminary studies of an experimental 9 wt. % Cr martensitic steel, exhibited very low oxidation rates under flowing steam at 650 °C for exposure times exceeding 20,000 h. A metallographic investigation at different time intervals has been carried out. Moreover, scanning transmission electron microscopy (STEM) analysis of a ground sample exposed to steam for 10,000 h at 650 °C revealed the formation of a complex tri-layered protective oxide comprising a top and bottom Fe and Cr rich spinel layer with a magnetite intermediate layer on top of a very fine grained zone.