MICROSTRUCTURAL AND MECHANICAL PROPERTIES OF MODIFICATED PRECIPITATION HARDENING STEEL 17-7PH AFTER MODIFICATION HEAT TREATMENT

Precipitation hardening stainless steel 17-7PH with modified chemical composition was heat treated by modified RH950 condition. In this paper is presented the results of tests of microstructure and mechanical properties of precipitation hardened stainless steel 17-7PH with modified chemical composition, heat treated in modificated RH950 conditions. Regression analysis showed which variables are statistically significant in predicting the value of mechanical properties of the steel 17-7PH chemically modified composition.

Precipitation hardening is generally achieved from homogeneously nucleated fine precipitates of intermetallic phases [8].

Semi-austenitic PH steels:
Precipitation takes place from the indirectly obtained martensite. These steels are austenitic in the solution annealed condition. They may contain from 5 to 20% of delta ferrite [2,6,9]. Semi-austenitic steels are ductile in a solution annealed condition and are easy to machined. The content of elements that extend alpha and gamma regions must be carefully balanced to obtain the desired properties of these steels. Cooling from solution annealing temperature must be fast enough to avoid grain coarsening [2,6].
The position of PH steel in the Schaeffler-Delong diagram is given in Figure 1 [2,7].

Information Of Testing Material: Chemical Composition:
Chemical composition of testing material and standard chemical composition for 17-7PH steel is given bellow. Modification of chemical composition for tested batches compared with the required chemical composition of 17-7PH steel is reflected in the content of chromium, nickel and aluminum, while the content of the five basic elements (C, Si, Mn, P and S) is within the limits prescribed in Table 1, and modification of chemical composition is ranged from 13.8 to 15.7% for chromium, for nickel from 7.3 to 9.1%, while aluminum content was from 0.61 to 1.53% [2].

Making experimental samples:
For the purposes of the experiment, sixteen experimental melts were made, with an ingot weight of 6 to 8 kg. The melts were produced in semi-industrial plant, in a vacuum induction furnace. Plastic processing of forging and rolling processed at semi-industrial plants. Forging was performed on 200 t hydraulic press and air hammer B250 with several warm-ups. Forging is completed after reaching a dimension of  18 mm. Hot rolling up to a final dimension of  16 mm was carried out on a semi-industrial line SKET rolling mill, with a roller diameter of 350 mm at a temperature interval of 1150 to 950 °C [2].

Modificated heat treatment RH950:
Rolled bars diameter 16 mm are heat treated solution annealing in a furnace without a protective atmosphere: heating to a temperature of 1050ºC for 115 minutes, holding at 1050°C for 30 minutes and cooled in air. A diagram of the solution annealing is given in Figure 2. The high strength of precipitation hardened 17-7PH stainless steel is achieved in the following steps: 1. conditioning of austenite: heating up to 955C for 90 minutes, holding at this temperature 10 minutes, 2. cooling below a critical temperature to transform austenite into martensite: cooling to air at room temperature, cooling to -50°C in dry ice within one hour and holding for 8 hours and heating to room temperature, 3. precipitation hardening: heating to 510°C for 45 minutes and holding at that 60 minutes and air cooling.
Diagrams of the heat treatment are given in Figure 3.

Testing Of Mechanical Properties In The Modified RH950 Condition:
The literature values of mechanical properties for 17-7PH stainless steel in condition RH950 and results of testing mechanical properties at elevated temperature for steel 17-7PH with modification chemical composition in modified RH950 condition are given in Table 2. A diagrammatic overview of the results of testing mechanical properties at elevated temperature is given in Figure 4. As microstructure show there are achieved the good percent of martensite, more than 80 percent and less than 5% of delta ferrite.

Regression Analysis Of The Effect Of Chromium, Nickel And Aluminum Content For The RH950 Condition:
Regression analysis of the results obtained by testing the mechanical properties after precipitation annealing at elevated temperature (Table 2), determined the correlation between the chromium, nickel, aluminum content and their interactions with independently variables -R m , Rp 0,2 and A.
The values of the correlation coefficients and the interrelation of independently variable chromium, nickel and aluminum, and their interactions for the modified RH950 condition are given in Table 3.  Index  variable   x6  x2  x3  x5  x1  x7  x4  ---A  0,0817  -0,379  0,536  -0,373  0,5598  0,4080  0,4359  1  Index  variable   x7  x5  x2  x6  x1  x4  x3  - Based on the Student's t-test values, it can be concluded that a statistically significant influence on the tensile strength R m in the modified RH950 condition at the test at elevated temperature shows the Cr content, while no chemical element or a combination thereof individually shows a statistically significant influence on the values Rp 0,2 and A, but all combined, over the observed chemical composition interval, have a statistically significant influence (the value of p and F for the model in the analysis of variance).