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Determination of the zero strength and plasticity of nickel alloys.

1. Introduction

Knowledge of the special features of the variation of the properties of metals at elevated temperatures expands the possibilities of improving their weld-ability and can be used to determine the conditions of decreasing the probability of cracking in the heat affected zone (HAZ) of the welded joints. It is believed that it is important to stress that the relationships of the variation of the mechanical characteristics of the metal, evaluated in modelling and investigation of the processes of solidification and phase transformations, can provide a large amount of information on the properties and structural condition of the metal and its welded joints, and also on the nature of hot cracking.

One of the criteria for the evaluation of sensitivity to hot cracking involving is the zero strength and zero plasticity of metal and temperatures close to the melting point. In cooling from the maximum temperature in the vicinity of the melting point of the plasticity remains equal to 0 in a certain temperature range until it is restored to higher values with further cooling. This temperature range, measured from the melting point, is referred to as the region of zero plasticity. The alloy susceptible to cracking in high-temperature deformation has a wide region of zero plasticity, whereas the alloy, susceptible to cracking, has a narrow region of zero plasticity [1, 2]. The latter explains the well-known dependence according to which the increase of the width of the temperature range [T.sub.liq]-[T.sub.sol] is accompanied by a reduction of the susceptibility to cracking [3].

2. Experimental procedure

To evaluate the processes taking place in the investigated nickel alloys at temperatures close to the melting point, investigations were carried out to determine the temperatures of zero strength and zero plasticity using the special method developed by the authors. The tests were carried out in argon. The specimens were heated by passing current through the standard specimens using flexible conductors, and the temperature was recorded with a tungsten-rhenium thermocouple. The specimens were loaded with a constant weight of 200 g to 80 kg by the method of testing for long-term strength.

The selected test procedure makes it possible to determine, in a wide temperature range, the dependence of the fracture temperature of the alloy on the level of loading. When the specimen fractures, the electrical circuit is interpreted and heating is stopped. The maximum recorded temperature is regarded as the temperature fracture for the given loading level. The zero strength temperature ([T.sub.z.str.]) is the temperature of fracture of the specimen without loading (the minimum load tends to 0).

The criterion for evaluating the strength properties was represented by the ultimate strength, determined from the equation:

[[sigma].sub.B] = P/s,

where P is the suspended weight, s is the cross-section of the specimen prior to testing.

The criterion for evaluating the plastic properties was the relative reduction in area [psi].

The tests were carried out on specimens of polycrystalline alloy ChS-70 and ZhS-26 alloy with a similar alloying system with directional solidification (Table 1).

3. The experimental results and discussion

The experimental results are presented in Figures 1 and 2. Analysis of the relationship of the variation of plasticity and strength at the temperatures of transition to the solid-liquid state shows an inflection point on the strength curve for both alloys. The temperature of this inflection point is identical with the temperature of the rapid decrease of plasticity to 0, indicating the start of melting at the grain boundaries for the polycrystalline metal and interdendritic zones for the alloy with directional solidification. Thus, the zero plasticity temperature ([T.sub.z.pl.]) characterises the process of the start of melting of the cast metal at high temperatures.

It should be mentioned that at the temperatures higher than the zero plasticity temperature, i.e., in the presence of a certain amount of liquid in the volume of the metal, the latter still has a certain strength. Evidently, this can be used to explain the inflection point on the temperature dependence of the strength at the start of the process of formation of the liquid phase.

These assumptions were confirmed in metallographic and fractographic investigations of the structure and constitution of the fracture surface of the specimens. As mentioned previously, the specimens of the ChS-70 alloy are characterised by the formation of an equiaxed cast structure with macrograins (Fig. 1a). The metal contains up to 50% of the hardening [gamma]'-phase, precipitated from the solid solution during cooling from the austenitisng temperature. A certain amount of the high-temperature carbide phases precipitate both in the body of the grain, evidently in solidification, and at the grain boundaries during subsequent cooling. The range between [T.sub.z.pl.] = 1185 and [T.sub.z.str.] = 1270[degrees]C is 85[degrees]C.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

The microstructure of the specimens of Zh-26 alloy consists mainly of the metal with the composition [gamma]/[gamma]', formed during cooling from the austenitising temperatures and eutectic colonies of the carbide phases (Fig. 2a). The range between [T.sub.z.pl.] = 1270 and [T.sub.z.str.] = 1295[degrees]C decreases and equals 25[degrees]C.

Discussing the variation of the mechanical properties on the basis of the strength and plasticity parameters in welding heating, it should be mentioned that they are linked closely with the structural changes in the nickel alloys and, in particular, with the behaviour of the [gamma]'-phase. The fragments of the specimens, prepared for evaluating the effect of high-temperature deformation on the microstructure (using the ZhS-26 alloy as an example), are presented in Fig. 3.

[FIGURE 3 OMITTED]

The investigations were carried out on the specimens fractured at a temperature of T = 1270[degrees]C situated between [T.sub.z.str.] and [T.sub.z.pl.]. In this case, cavities were found in the melted areas (Fig. 3a). The specimens fractured at the temperature T = 1055[degrees]C of rapid dissolution of the particles of the [gamma]'-phase (Fig. 3b) and the sections of the specimen which were not heated were also investigated.

The microstructure was studied in a scanning electron microscope on sections with the surface etched by the chemical method. In addition, the distribution of the main alloying elements between the components of the structure was evaluated on the unetched surface of the sections.

Investigations were carried out into the structure and properties of the metal at room temperature (Figures 4 and 5) and in the immediate vicinity of the fracture area at temperatures of 1270 (Fig. 6, 7) and 1055[degrees]C (Fig. 8). This procedure makes it possible to estimate the structural state in nonuniform heating of the metal in actual welding from the melting point at the diffusion line to moderate temperatures away from the weld pool.

[FIGURE 4 OMITTED]

The experimental results also show that the structure of the parent metal of the ZhS-26 alloy, not subjected to heating to high temperatures, consists mainly of the austenitic matrix and the precipitates of the [gamma]'-phase with a shape similar to the cubic four-lobe shape (Fig. 4d); this is in agreement with the literature data [4]. The presence of chemical dendritic heterogeneity in solidification of the Zh-S-26 alloy, including the distribution of the main [gamma]'-forming elements Al and Ti, results in the formation of the [gamma]'-phase of different sizes. The dimensions of the [gamma]'-phase in the centre of the dendrite (Fig. 4d) are considerably smaller in comparison with the areas of intergrowth of the dendrite enriched with aluminium and titanium (Fig. 4e). In addition to the [gamma]/[gamma]'-components, the structure contains a certain amount of the plate-shaped carbide eutectic.

The composition of the elements, included in the eutectic, is shown in Fig. 5. The graphs indicate that the plate-shaped phases are enriched with carbon (spectra 5, 6), niobium and tungsten. The spherical particles of the irregular shape (spectra 3, 4) are greatly enriched with C, Nb, W and evidently represent the carbides of the type MeC, (Nb, W) C. These carbides are high-temperature carbide which evidently do not impair the high-temperature strength characteristics of the metal although they reduce the plasticity of the metal.

Investigations were carried out into the zone of high-temperature heating of the specimen, fractured at T = 1270[degrees]C, in the vicinity of the fracture area. The results, obtained in a scanning electron microscope for the etched surface of the cross-section of the given specimen, are shown in Fig. 6. In contrast to the initial structure produced as a result of heat treatment, it was not possible to obtain at comparable magnification (x10,000) the distinctive separation of the [gamma]- and [gamma]'-phase which indicates that the size of the [gamma]'-phase is very small. The results are confirmed by the data obtained in investigations of the welding of nickel superalloys [5, 6] according to which the size of the [gamma]'-phase at a high cooling rate, comparable with the cooling rate of the heat affected zone, is smaller than 40.0-60.0 nm. This value is greater than the resolution power of the scanning microscope and, consequently, it is not possible to detect the structure of the [gamma]'-phase.

[FIGURE 6 OMITTED]

In addition to the main structure [gamma]/[gamma]', the specimens showed melting of the interdendritic zones. The elemental composition of the structural components, carried out in an electron scanning microscope using an energy-displacing x-ray analyser, shows the enrichment of the melting zones with carbon and the elements characterised by strong affinity for carbon, such as W, Nb, Mo, Cr, Ti (spectrum 4) (Fig. 7).

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

It is interesting to evaluate the structure of the metal of the specimen heated by passing current to 1055[degrees]C, i.e., to the temperature of the high-rate [gamma]' [right arrow] [gamma]-transformation followed by rapid cooling. Figure 8 shows the fragment of the surface of the specimen at different identification (maximum magnification x10 000). Analysis of the surface makes it possible to indicate the remnants of the primary [gamma]'-phase which did not dissolve during heating in the form of four-sided lobes. Evidently, the area between the undissolved primary [gamma]'-phase is occupied by the [gamma]/[gamma]' structure in which the particles of the [gamma]'-phase, formed as a result of rapid cooling, have the size which can be determined using the given experimental techniques.

Thus, the structure of the specimen consists of the relatively large precipitates of the primary [gamma]'-phase and the secondary [gamma]/[gamma]'-phase. This clarification in the formation of the structure is essential for determining structural changes in the heat affected zone of the welded joints where the heating and cooling time is comparable with the heating time in the evaluation of the zero strength and plasticity of the metal.

Fractographic investigations were carried out on the fracture surface of the specimens, produced in the plasticity and strength tests at temperatures of T = 1055[degrees]C and T = 1270[degrees]C (Fig. 9).

The specimen fractured at 1055[degrees]C (Fig. 9a-d) is characterised by brittle fracture. Fracture takes place mainly through the plate-shaped precipitates of the eutectic carbide phases. The specimen tested at 1270[degrees]C fractured at the moment of existence of the solid-liquid state (Fig. 9d-e). This is indicated by the remnants of the molten dendrites and also by the smooth surface formed under the effect of surface tension forces.

Thus, the evaluation of the temperatures of zero strength and zero plasticity of the alloyss makes it possible to evaluate the weldability of the materials, including the sensitivity to hot solidification cracks by evaluating the size of the temperature range between [T.sub.z.str.] and [T.sub.z.pl.]. For the alloys characterised by the minimum difference of the temperature of zero strength and zero plasticity (minimum high-temperature range of the plasticity dip) the duration of existence and thickness of the liquid interlayer at the grain boundaries is minimum and, consequently, these materials are characterised by high resistance to solidification cracking.

4. Conclusions

The specially developed method of determination of the temperatures of zero strength and zero plasticity in heating specimens by the passing current under the effect of small loading of creep-resisting nickel alloys has confirmed the rapid decrease of plasticity to zero whilst maintaining some reserve of the strength in the temperature range close to the melting point. This indicates the existence of the solid-liquid state at the grain boundaries for the polycrystalline alloy and at the interdendritic zones, enriched with W, Nb, Mo, Cr, Ti, C for the alloy prepared by directional solidification.

An increase of the range between the temperatures of zero strength ([T.sub.z.str.]) and zero plasticity ([T.sub.z.pl.]) increases the duration of existence and the thickness of the liquid interlayer at the grain boundaries. Consequently, the probability of cracking in the welded joint is higher.

Metallographic examination of the specimens, tested at T = 1055[degrees]C, characterised by the occurrence of the high rate [gamma]' [right arrow] [gamma]-transformation, has made it possible to separate the elements of the undissolved [gamma]'-phase in the form of four-sided lobes.

The fine, newly formed particles of the [gamma]'-phase appeared between the particles of the residual y'-phase. The metallographic investigation of the specimen tested at the temperature of T = 1270[degrees]C, i.e., between [T.sub.z.pl.] and [T.sub.z.str.] shows the presence in the melted areas of discontinuities which may become the reason for the formation of solidification cracks.

References

(1.) Ch. G. Sims et al (editors), Superalloys, in two volumes, Metallurgiya, Moscow, 1995.

(2.) Bo Roberg, Scand. J. Met., 12, No. 2, 51 (1983).

(3.) K. A. Yushenko and V. S. Savchenko, Hot Cracking Phenomena in Welds (Horst Herold, Springer Verlag, 2005).

(4.) B. E. Paton, et al., Creep strength of cast nickel alloys and oxidation protection, Naukoda dumka, Kiev, 1987.

(5.) M. H. Haafkens and J. H. Matthey, Welding Journal, 61, No. 11, 25 (1982).

(6.) K. Yushchenko, Materials of International Institute of Welding. General Assembly (Lisbon, Portugal, 1999) (Lisbon: IIW: 1999), p. 82.

K.A. Yushchenko, V.S. Savchenko and A.V. Zvyagintsev E.O. Paton Electric Welding Institute, Kiev
Table 1. Chemical composition of nickel creep-resisting alloys with
[gamma]'-hardening

Alloy                 Mass fraction of elements, %
         C      Cr      Co     Al        Ti        Mo        W

ZhS-26   0.15   5.0     9.0    5.5-6.2   0.8-1.2   0.8-1.5   1-13
ChS70    0.14   15.74   11.0   2.8       4.2       2.12      4.9

Alloy                 Mass fraction of elements, %

         Nb    V         B       Si    Fe     Mn     Hf    Ni

ZhS-26   1.6   0.8-1.2   0.015                             Base
ChS70    0.5   --        --      0.1   0.34   0.03   0.3   Base

Alloy    Amount of
         [gamma]'-phase, %

ZhS-26   65
ChS70    50

Fig. 5. Local areas of chemical microheterogeneity.

Results in wt.%

Spectrum 1        C       Al     Ti      V      Cr     Co     Ni

                          5.31    0.9    1.05   5.27   9.12   62.72
Spectrum 2                5.63    1.33   0.84   5.06   8.92   63.86
Spectrum 3        17.37          11.56   1.61   0.4            2.55
Spectrum 4        15.48   0.4    11.44   1.79   0.98   1.14    8.93
Spectrum 5         8.39   2.84    4.24   1.81   2.96   4.76   38.01
Spectrum 6         3.16   3.66    3      1.68   4.58   7      48.83
Spectrum 7         3.94   4.82    1.67   1.16   4.51   8.09   58.15
Spectrum 8                7.69    1.73   0.82   2.66   7.57   70.6

Spectrum 1        Nb      Mo     W       Re

                   1.19   1.47   12.36   0.62
Spectrum 2         1.31   1.36    9.98   1.7
Spectrum 3        46.68   3.23   16.07   0.54
Spectrum 4        38.54   2.41   18.39   0.52
Spectrum 5        12.68   2.62   20.75   0.93
Spectrum 6         7.42   1.85   17.15   1.67
Spectrum 7         3.01   1.5    12      1.15
Spectrum 8         2      0.41    6.53

Fig. 7. Distribution of chemical elements between the structural
constituents of ZhS-26 alloy, tested at 1270[degrees]C.

Determination of zero strength and plasticity

Results in wt.%

                  C      Al     Ti     V      Cr      Co     Ni

Spectrum 1               4.99   0.82   0.94    4.72   9.33   63.26
Spectrum 2               4.66   0.74   1.01    4.8    9.55   63.49
Spectrum 3        2.35   5.74   1.88   0.88    5.1    7.94   64.9
Spectrum 4        3.91   2.29   1.67   1.57   11.09   8.74   41.08
Spectrum 5        3.57   3.39   1.76   1.4    10.15   8.54   48.92
Spectrum 6        3.59   4.75   1.65   1.28    5.36   7.58   58.06
Spectrum 7        3.11   5.17   1.96   1.28    5.48   7.92   58.46
Spectrum 8        2.08   5.94   1.75   1.16    5.62   8.49   64.38

                  Nb     Mo     W       Re

Spectrum 1        0.57   1.03   12.88   1.46
Spectrum 2        1.07   1.29   13.38
Spectrum 3        2.73   1.11    7.38
Spectrum 4        8.18   6.61   14.86
Spectrum 5        5.51   4.92   11.85
Spectrum 6        3.71   1.63   11.45   0.95
Spectrum 7        3.73   1.18   10.75   0.96
Spectrum 8        2.12   1.05    7.41
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Title Annotation:PHYSICS OF STRENGTH AND PLASTICITY
Author:Yushchenko, K.A.; Savchenko, V.S.; Zvyagintsev, A.V.
Publication:Physics of Metals and Advanced Technologies
Article Type:Report
Geographic Code:4EXUR
Date:Jan 1, 2010
Words:2918
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