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Maraging steels are a special class of ultra-high strength steels. These steels are used in a variety of applications with excellent mechanical properties, good corrosion resistance and simple process of heat treatment [1-4]. Maraging steels are widely used in applications such as military and commercial industries, mainly for aircraft, tooling applications [1, 2]. Instead of relying on carbide precipitation for a traditional method, these steels are hardened by the precipitation of intermetallic compounds. Due to the absence of carbon in the steels, it confers better formability, hardenability, and a combination of strength and toughness .
Maraging refers to the ageing of martensite which is a hard microstructure commonly found in steels. Martensite is easily obtained in these steels which have high nickel content. The martensite formation that occurs at common cooing rates is the only transformation. The absence of carbon makes the martensite quite soft, but heavily dislocated. Hardening and strengthening of these steels under heat treating for several hours at 480-510°C are produced, caused by precipitation. The research and development of maraging steels had focused on the precipitation process and identification. Maraging steels consist of a large number of alloying elements which are expensive materials compared with many other engineering alloys. The development of maraging steels is significantly influenced by the availability and price of the alloying elements. The ageing heat treatment process for the manufacturing of maraging of maraging steels is very simple.
The commercial maraging steels contain high levels of nickel, cobalt and molybdenum. Original development was carried out on steels with high level of nickel since it was a principal alloying element in maraging steels. According to the different nickel contents, maraging steels are divided into different types. Due to the simple process of production and stable performance, the 18Ni maraging steels are the most widely used. Therefore, the 18Ni maraging steels are in an advanced and mature stage of development and applications, with the maximum strength levels with good toughness and ductility reaching 2400 MPa . However, these steels contain high level of cobalt as high as 8-13%. Since cobalt is an expensive alloying element, this keeps the steels quite expensive and preventing wider selection and application. Therefore, developing cobalt-free maraging steels with reduced quantities of expensive alloying elements in order to lower the production cost had been an important direction of maraging steels research. Similarly, nickel has been widely used in maraging steels, but the high cost of nickel demands a second thought of the actual amounts required of this element in these steels. Therefore, research to develop novel maraging steels of reduced nickel content, for high strength applications with good toughness at reduced steel should be attempted.
Approximate cost per tonne
Table 1.1: Cost competitiveness assessments [From figures given by Prof. Sha]
The compositions of existing and experimental maraging steels, the cost per tonne are illustrated in Table 1.1. Comparing the recent prices of steels per tonne in UK, it is obviously shown the 12% nickel steels obtain the lowest price per tonne. If there are no significant differences in properties among them, the lowest price one absolutely will be the most attractive steels. The study on developing novel maraging steels with reduced nickel content, obtain almost equivalent properties at a reduced cost may make sense.
1.2 Aims and Objectives
The main aim of this project is to develop novel maraging steel with reduced nick content, for high strength applications with good toughness at a reduced steel cost. The objectives are to complete laboratory-scale mechanical testing and microstructural characterisation of a grain refined maraging steel of reduced nickel content. The mode of brittle fracture needs to be determined. Also quantification of age hardening can be carried out.
Chapter 2: Literature Review
Heat treatment is used to alter the physical and chemical properties of a material. As the most common application on metallurgical, it involves the use of heating or chilling to achieve a desired result such as hardening or softening of material. Annealing, precipitation hardening, tempering and quenching are commonly employed techniques.
Precipitation hardening is one of the most effective methods in the development of ultrahigh-strength alloys. It is worked by producing a particulate dispersion of obstacles to dislocation movement, using a second phase precipitation process.
Precipitation hardening or age hardening is a heat treatment technique used to enhance the strength of maraging steel. Since it is worked by producing a particulate dispersion of obstacles to dislocation movement, this serves to harden the material. Precipitation in steels can produce many different sizes of particles, which have different properties.
Two different processes result in maraging steels contain small amounts of austenite. First, after cooling, the material can display a small quantity of retained austenite from the austenite phase region, that is, after the solution treatment. Second, on the period of ageing, a partial reversion from martensite to austenite can occur [5, 6], the different ageing time can result in the amount of reverted austenite.
Many studies on microstructure characterization and its influence on the mechanical properties of maraging steels have been conducted [1, 7-15]. In general, the intermetallic precipitates which form in the ageing process result in the enormous increase in hardness and strength .
It is understood that the austenite phase in maraging steels is not stable during deformation [10, 16-18], which lead to a transformation to martensite.
Development of Precipitation Hardening
It was first discovered by Wilm in aluminium alloys . Since then, precipitation strengthening mechanism and precipitation kinetics became the subject of researches . However, it was not had a basic understanding of age hardening really been achieved until the introduction of the concept of dislocation by Mott and Nabarro . A landmark achievement was done since Orowan derived the equation several decades later . It remains the basis for the theory of dispersion strengthening. A review by Kelly and Nicholson studied the early attempts at formulating theories of precipitation hardening . Brown and Ham studied the developments of ways in dislocations interact with precipitates . Later, the understanding of ageing hardening of the statistics of dislocation-particle interactions and the mechanism of ageing hardening were discussed by Ardell . Recent years, the mechanisms of hardening are discussed fully [26, 27] or partially [28, 29], follow what had been discussed by Ardell.
The Mechanism of Precipitation Hardening
The increase of the precipitate particles size is influenced similarly by precipitates growth and coarsening. However, they have different effects on the properties in metal. The hardness of material is normally enhanced by precipitates growth but reduced by precipitates coarsening. The main differences between them are in diffusion routes and the distance of diffusion field (DoDF) . In the aspect of nucleation routes, for precipitate growth, the stable phase of precipitates increases at the cost of the transition phase which is formed early. For precipitates coarsening the difference of concentration between the smaller particles and bigger particles triggers the movement of smaller particles to bigger particles. In term of distance of diffusion field (DoDF) exhibiting pronounced difference compared to two factors, the DoDF reduces in precipitates growth, and the reducing rate depends on the amount of nucleation site. In contrast, precipitates coarsening always results in the increase of the average DoDF.
Fig. 2.1: Schematic comparison in diffusion routes between (a) Growth and (b) Coarsening .
In stage 1, the increase of strength is nearly proportional to the ageing time at beginning, and become slower near the peak value. This stage is described as underaging, and is usually called strengthening, mainly due to the resistance of precipitates to dislocation cutting. Ager prolonged ageing, the strength pasts the peak value and decreases subsequently. This period is denoted as overaging, or softening, as shown in the stage 2. The decrease of strength in stage 2 is attributed to dislocations forced to loop around the precipitates. However, it is noted that strengthening and softening can both increase the strength compared with the material without ageing.
The development of low nickel content maraging steels has been accelerated strongly so far, mainly due to its cost-competitiveness. Table 1.1 shows the approximate alloying costs per tonne for two existing and two experimental maraging steels (Sha et al., 2011). The standard 18% nickel steel with cobalt is the most expensive and followed by the nickel steel with nickel content around 18 wt%. The 12% nickel steels will save around £800 comparing with the cobalt free 18% nickel steel. Hence, low nickel content maraging steels should be more cost-competitive in market.
As seen in Fig. 1, two stages are in a typical one-peak precipitation-strengthening curve. In Stage 1, the resistance of a precipitate against dislocation cutting results in strength increase. In Stage 2, a dislocation is forced to loop around the precipitate rather than cutting through, which also results in strength increase compared with the solution-treated material. To make the discussion easier, strengthening is used to describe stage 1 - the underaging period, and softening is used to describe stage 2 - the overaging period.
Fig. 2.2: A typical one-peak age hardening curve .
A great amount of research has been carried out over the years on the ageing microstructure, mechanical properties and strengthening mechanisms of maraging steels. During ageing treatment, the dense, fine and complex microstructures can be appeared in maraging steels, with precipitates having complicated diffraction patterns .
Chapter 3: Experimental Procedure
3.1 Experimental Materials
The maraging steel of reduced nickel content with a composition of Fe-12.94Ni-1.61Al-1.01Mo-0.23Nb-0.046C (wt%) was investigated in this experiment. The steel material was vacuum melted at Ross & Catherall, Killamarsh, Sheffield, UK. To homogenise and break up the as cast structure, the maraging alloy was upset forged at Stockbridge Engineering Steel, UK, from 70 mm diameter billet Ã- ~170 mm in height to 25 mm thick disc Ã- 145 mm diameter at 1200 °C, followed by air cooling to room temperature.
Standard samples, with the size of 10Ã-3Ã-4mm made by the mechanical workshop in Ashby (Queen's university), were machined and used. The ageing was carried out in furnace with a linear rate of 30 Kmin-1 and the samples were set into it when the desired temperature was reached.
3.2 Experimental Methods
Small samples from the steel were subject to ageing treatment at 575 °C, for times near 10 min, 25 min, 1 h, 2 h, 4 h, 8 h, 16 h, 24 h, 48 h, 72 h and 120 h. The samples were hearted uniformly in a furnace, and the oxidised surface layer was removed by cutting. Then the conventional metallographic procedures of sample preparation including mounding, grinding, polishing, and etching were conducted.
For optical and scanning electron microscopy, a typical etching procedure was carried out using a solution of 5% HNO3 acid in ethanol, to reveal the grain boundaries and precipitates. The etching time was 5-10 seconds.
The microstructures were observed by optical microscopy and scanning electron microscopy (SEM). Precipitates were examined and the size of the precipitates was compared after the ageing treatment.
Hardness curve with a 2 kg load were conducted, using a microstructure machine. Impact test were used for measuring the toughness of the material, followed by fractography at room temperature. Sub-standard half-size V-notched specimens were used, 5Ã-10Ã-55 mm. For testing above room temperature, the specimens were placed in air circulating oven at intended testing temperatures for 30 minutes, and were transferred to Charpy machine. The Charpy impact specimens were made and the tests were carried out by Sheffield Testing Laboratories, in accordance with BSEN 10045-1 and ISO 6507-1.
3.3 Risk Assessment
Hazardous Material Involved:
Solution of 5% nitric acid is an irritant to the eye, skin, respiratory tract and the digestive tract. It can be damage the eyes. Can be harmful if inhalation, ingestion or adsorption by the skin causing dizziness, headache.
All solution preparation will be performed in a fume hood cupboard. Wear appropriate protective eyeglasses or chemical safety goggles. Wear appropriate protective gloves to prevent skin exposure. Wear appropriate protective clothing to prevent skin exposure.
Waste Disposal Procedure:
All liquid waste will be diluted prior to disposal and poured to sink. The solid waste like broken samples will be placed in plastic bag and labeled date and heat treatment parameters, intending to return to the supervisor for recycling.
Emergency Procedures (acid only):
Spillage: Absorb spill with inert material (e.g. vermiculite, sand or earth) then place in suitable container. Clean up spills immediately. Provide ventilation.
Fire: If there is a fire, the extinguishing media have to be selected to suit the materials in the surrounding areas.
Skin contact - flush with copious amount of water for at least 15 minutes. Remove contaminated clothing and wash before reuse. Unless contact has been slight obtain medical attention.
Inhalation - remove to fresh air, rest and keep warm. If breathing is difficult give artificial respiration and obtain medical attention.
Eye contact - irrigate with copious amounts of eye wash or water for at least 15 minutes. Assure adequate flushing by separating the eyelids with fingers. Obtain medical attention.
Ingestion - do not induce vomiting. If conscious provide water for person to thoroughly wash out mouth (and sip if required). Obtain medical attention.
3.4 Laboratory Equipment
Fig. 3.1: Mounting press.
This machine is used to encapsulate samples for metallographic preparation. Time of heating and cooling which are mounting cycle parameters can be preseted. At the end of each cycle, the cooled sample can be pulled out from the heating chamber and passed onto the polishing process.
Fig. 3.2: Manual polishing machine.
Polishing is worked by using many abrasive papers begin with Grit 250, then Grit 400, Grit800, Grit 1200 and Grit 2400. Samples were kept checking with optical microscope after polishing.
Fig. 3.3: Samples and Optical microscope for polishing check.
Fig. 3.4: Hardness testing machine
Hardness is a measure of resistance of material to deformation when an external force or load is applied to the material. The hardness scale is Vickers hardness (HV).
Fig. 3.5: Optical microscope with image capture.
The microstructure was investigated by optical microscopy.
Fig. 3.6: SEM (Scanning electron microscope)
SEM was used to observe the microstructure. Using the electron microscopy techniques, precipitates were examined and the size of the precipitates was compared during the ageing treatment more clearly.
Chapter 4: Experimental Results & Discussion
The hardness test results for samples after ageing at 575 °C for different time times are illustrated in Fig. 4.1. A strong influence of the ageing time on the curve can be clearly seen. The ageing temperature has led to rapid hardening response. As expected, a significant increase of hardness is shown after ageing. The hardness reaches its peak when the ageing time is 1 h, with the maximum hardness of 459 HV2. Moreover, the hardness increase and decrease rate is marginally from 10 min to 2 h. The hardness displays a large plateau around the peak hardness. The hardness test results show that hardness decrease not long after it reaches the peak.
Fig. 4.1: Age hardening curve, showing the variation of hardness, with ageing time, in the maraging steel aged at 575 °C.
Fig. 4.2: Age hardening curves, showing the variation of hardness, with ageing time, in the maraging steel aged at 450-600 °C. [From papers given by Prof. Sha]
There is a close relation between hardness and precipitation in maraging steels. The hardness data in Fig. 4.2 shows age hardening curves of the maraging steel at different ageing temperatures. When aged the lowest temperature of 450 °C, the steel can attain the average hardness of 401 HV2 after 1 h ageing. Both 550 and 600 °C ageing temperature have led to rapid hardening responses. At 600 °C ageing temperature, the hardness reaches its peak when the ageing time is 0.25 h, with the maximum hardness of 467 HV2, followed by slow reduction to 301 HV2 after ageing for 257 h. It takes 2 h for the steel at 550 °C ageing temperature to reach the peak hardness of 496 HV2. The hardness increase rate is marginally slower than at 600 °C ageing temperature. Ageing at 450 °C gives the lowest hardness increase rate, and requires the longest time of 66 hours to reach the peak hardness 500 HV2. When aged at 450 °C, the hardness keeps increasing up to and likely after the longest ageing time used at this temperature. Moreover, at the higher ageing temperature 500 °C, the maximum ageing hardness is the same as the maximum hardness at 450 °C, within error ranges. The peak hardness is 501 HV2 at 17.35 h.
When compare the curve ageing at 575 °C with the curve at 550 °C and 600 °C respectively. All of the three curves have led to rapid hardening responses and the curve shapes tend to be similar. The steel ageing at 575 °C ageing temperature gives the lowest peak hardness of 459 HV2 when the ageing time is 1h. It takes 2 h for the steel at 550 °C ageing temperature to reach the peak hardness of 496 HV2. At 600 °C ageing temperature, the hardness reaches its peak when the ageing time is 0.25 h, with the maximum hardness of 467 HV2. It seems that shorter time at higher temperatures and longer time at lower temperature. The results illustrate that similar peak can be reached at all three ageing temperatures, but the time of reaching the peak hardness is different. Moreover, the peak hard is not always increased by the positive relationship. The overageing happens to the steel relatively early when aged at the relatively high temperature.
The microstructure of the steel after etching is shown in Fig. 4.3. The martensite laths and the grain boundaries are revealed. Few dark areas are revealed after ageing for a long time at 575 °C.
It is noted that the martensitic transformation is weakly affected by the variation of the prior austenite grain size . Therefore, small martensite laths are not transformed by small austenite grains, i.e. the size of martensite is nearly changeless while prior austenite grain size is refined.
Austenite is known as a kind of soft phase, which can provide little strength to steel. For martensite, the hardness is mainly from the high density dislocation in the martensite lath. Therefore, it can be assumed that the refinement of prior austenite grain size may not significantly change the density dislocation in the martensite lath. Thus, the result is nearly unchanged hardness.
Fig. 4.3: Optical micrographs before and after ageing. (a) As-forged; (b) 575 °C, 1 h; (c) 575 °C, 72 h; (d) 575 °C, 120 h. The four micrographs have a same magnification. [Micrograph (a) is from papers given by Prof. Sha]
Fig. 4.4: Scanning electron micrographs after ageing. (a, b, c, d) 575 °C, 120 h at increasing magnification; (e) 575 °C, 10 min; (f) 575 °C, 0.
The martensite laths and the grain boundaries are more clearly revealed in SEM images, as shown in Fig. 4.4. Fine precipitates can be found homogeneously lying on the surface of Fig. 4.4 (c).
Fig. 4.4 (a, b, c, d) shows the micrographs of samples ageing at 575 °C for 120 h at increasing magnification. Fig. 4.4 (d, e, f) illustrates the micrographs of sample at the same magnification with sample (d) ageing at 575 °C for 120 h, sample (e) ageing at 575 °C for 10 min and sample (f) without ageing, respectively. Since such a austenite fraction were not detected by X-ray diffraction analysis, the darkening in Fig. 4.4 (d, e) does not seem to be related to austenite. The many precipitates observed should be the martensite. It is obviously shown that the precipitates in Fig. 4.4 (d) are larger than in Fig. 4.4 (e). A small amount of precipitates can be observed in Fig. 4.4 (f) as well though the sample without ageing. These precipitates are consisted of different types of intermetallic phases. Few precipitates can be found in Fig. 4.4 (e) than in Fig. 4.4 (d). The distribution of precipitates is not homogeneous in Fig. 4.4 (e) while comparing with the precipitates in Fig. 4.4 (d). Therefore, this is probably a result of the longer ageing time treatment.
With increasing ageing time, the yield and the tensile strengths decrease rapidly, while the elongation increases continuously. The reduction in area is maintained at quite high levels. From the variations of the yield and tensile ratio decreases with decreasing strength from 0.96 to 0.85. This is in sharp contrast to the independency on ageing time of the yield to tensile ratio when aged at low and medium temperatures.
After high temperature ageing, the fracture toughness of the reduced nickel maraging steel does not reach the levels of other maraging steels at comparable strength grade.
Fig. 4.5: Charpy impact energy as a function of specimen temperature at testing. Steel aged at 550 °C for 10 h. [From papers given by Prof. Sha]
Steels undergo a transition in fracture behaviour from brittle to ductile with increasing temperatures are shown in Fig. 4.5. The absorbed energy (Joule) is plotted against testing, giving a ductile to brittle transition temperature cure (DBTT curve). The curve represents a change in fracture behavior from ductile at high temperature to brittle at lower temperature. The lower shelf presents a brittle fracture, the transition-mixed mode presents a mixed mode of brittle and ductile fracture, the upper shelf presents a ductile fracture.
Fig. 4.6: Fractographs of impact specimens (aged at 550 °C for 10 h) tested at different testing temperature. (a) -40 °C, 3 J; (b) RT, 4 J; (c) 75 °C, 13 J; (d) 150 °C, 10 J; (e) 250 °C, 17 J; (f) 300 °C, 13J. The six micrographs have the same magnification.
SEM fractographs of impact specimens tested at different testing temperature are shown in Fig. 4.6. The fractographs show cleavage facets at -40, RT, 75 °C in Fig. 4.6 (a), Fig. 4.6 (b) and Fig. 4.6 (c), respectively. The fracture surface shows a mixed morphology, cleavage facets and ductile tearing in Fig. 4.6 (d) though the elongation is very large. The fractographs show small and deep tensile dimples, with small precipitates at the bottom of many dimples at 250, 300 °C in Fig. 4.6 (e) and Fig. 4.6 (f), respectively. Therefore, the SEM fractographs of specimens tested at -40, RT, 75 °C should be typically brittle fracture process, at 150 °C should be mixed mode of brittle and ductile fracture process at 250, 300 °C should be ductile fracture process.
As seen in Fig. 4.6, the fractographs of impact specimens at higher temperature has bigger voids and more dense inclusions than the lower. These features basically show the same tendency as shown in other fractographs. With the temperature increasing, the fracture surface becomes bumpier. The tendency of fracture is from the cleavage fracture to intergranular separation. They are both clearly seen in fractographs. Fractograph obtained from specimens tested at 150 °C show increased bumpy and blocky fracture surfaces in Fig. 4.6 (d).
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(c)Fig. 4.7: SEM fractographs of impact specimens (aged at 550 °C for 10 h) tested at 150 °C, at increasing magnification; (a, b, c) 150 °C.
As seen in Fig. 4.7, they all exhibit intergranular separation along inclined surfaces. Higher magnification shows plateau or cavity images in which both cleavage and intergranular separation are seen in which the inclined surface image has tiny cleavage surfaces imbedded in a ductile fracture surface. It shows that intergranular separation occurs by ductile fracture with dimples.
The decrease of impact energy toward lower temperature is sharp. The low impact energy of the specimens at RT seems to be caused by both intergranular separation and cleavage fracture.
Chapter 5: Conclusion
The study shows the age hardening behaviour of the maraging steel. Investigation of the microstructure and fractography of the steel with reduced nickel has been carried out. From the conducted experiments, the main conclusions are as follows:
The age hardening rate across the ageing treatment temperatures of 550-600 °C is higher than 450-500 °C.
There are no significant changes of precipitations.
The steel is tough before ageing, but extremely brittle at room temperature after ageing.
Intergranular separation may involve the deformation of grain boundary austenite.
Brittle fracture occurs in two modes, transgranular cleavage and intergranular separation.