Section 6 Forgings for turbines

6.1.1 Provision is made in this Section for ferritic steel forgings for turbine rotors, discs and spindles, turbine-driven generator rotors and compressor rotors.

6.1.2 Plans for rotor forgings are to state whether the rotor is intended for propulsion or auxiliary machinery and the shaft power of auxiliary turbines. In the case of a rotor which is to be tested for thermal stability, the maximum operating temperature and the proposed test temperature are also to be stated.

6.1.3 Specifications of alloy steel forgings giving the proposed chemical composition, heat treatment and mechanical properties are to be submitted for approval with the plans of the components.

6.1.4 Where it is proposed to use rotors of welded construction, the compositions of the steels for the forgings are to be submitted for special consideration, together with details of the proposed welding procedure. Welding procedure tests may be required.

6.2.1 Forgings are to be manufactured in accordance with the requirements of Ch 5, 1 General requirements, except that for rotors the forging reduction is to be not less than 2,5 to 1. Where an upsetting operation is included in the manufacturing procedure, the above requirement applies to the cross-sectional area of the upset bloom and not to that of the ingot.

6.3.1 The chemical composition of ladle samples is to comply with Ch 5, 3.2 Chemical composition 3.2.1 for carbon and carbon-manganese steels and Ch 5, 1.4 Chemical composition 1.4.3 for alloy steels.

6.4.1 Forgings are to be supplied in the heat treated condition, and the thermal treatment at all stages is to be such as to avoid the formation of hair-line cracks. At a suitable stage of manufacture, the forgings are to be reheated above the upper critical point to refine the grain, cooled in an approved manner and then tempered to produce the desired mechanical properties.

6.4.2 Where forgings receive their main heat treatment before machining, they are to be stress relieved after rough machining. Forgings which are heat treated in the rough machined condition need not be stress relieved provided that they have been slowly cooled from the tempering temperature.

6.4.3 The tempering and stress relieving temperatures are to be not less than 550°C for carbon and carbon-manganese steels, and not less than 600°C for alloy steels. The holding times and subsequent cooling rates are to be such that the forging in its final condition is free from harmful residual stresses.

6.4.4 Details of the proposed heat treatment for rotors of welded construction are to be submitted for approval.

6.5.1 At least one tensile test specimen, cut in a longitudinal direction, is to be taken from each rotor forging. For forgings exceeding both 3 tonnes in mass and 2000 mm in length, tests are to be taken from each end.

6.5.2 For rotor forgings of all main propulsion machinery and of auxiliary turbines exceeding 1100 kW, tangential and, where the dimensions permit, radial tensile tests are to be taken from the end of the body corresponding to the top end of the ingot, see Figure 5.6.1 Test positions for turbine rotor forgings.

Figure 5.6.1 Test positions for turbine rotor forgings

6.5.3 For each turbine disc, at least one tensile test specimen is to be cut in a tangential direction from material at the hub, see Figure 5.6.2 Test positions for turbine disc forgings.

6.5.4 For the tests required by Ch 5, 6.5 Mechanical tests 6.5.1 to Ch 5, 6.5 Mechanical tests 6.5.3, sufficient test material is to be left on each forging and is not to be removed until all heat treatment, including stress relieving, has been completed. In this connection, a thermal stability test does not form part of the heat treatment of a turbine forging. Any excess test material is not to be completely severed from a forging until all the mechanical tests have been completed with satisfactory results.

Figure 5.6.2 Test positions for turbine disc forgings

6.5.5  Table 5.6.1 Mechanical properties for acceptance purposes: carbon-manganese steel forgings for turbines - Normalised and tempered and Table 5.6.2 Mechanical properties for acceptance purposes: alloy steel forgings for turbines - Quenched and tempered or normalised and tempered give the minimum requirements for yield stress, elongation and reduction of area corresponding to different strength levels, but it is not intended that these should necessarily be regarded as specific grades. The strength levels have been given in multiples of 40 N/mm², or 50 N/mm² for alloy steels, to facilitate interpolation for intermediate values of specified minimum tensile strength.

6.5.6 Forgings may be supplied to any specified minimum tensile strength selected within the general limits detailed in Table 5.6.1 Mechanical properties for acceptance purposes: carbon-manganese steel forgings for turbines - Normalised and tempered or Table 5.6.2 Mechanical properties for acceptance purposes: alloy steel forgings for turbines - Quenched and tempered or normalised and tempered.

6.5.7 The results of all tensile tests are to comply with the requirements of Table 5.6.1 Mechanical properties for acceptance purposes: carbon-manganese steel forgings for turbines - Normalised and tempered or Table 5.6.2 Mechanical properties for acceptance purposes: alloy steel forgings for turbines - Quenched and tempered or normalised and tempered appropriate to the specified minimum tensile strength. For monobloc rotor forgings, the specified minimum tensile strength is not to exceed 800 N/mm².

6.6.1 The end faces of the body of rotor forgings and the end faces of the boss and the bore surface of each turbine disc are to be machined to a fine smooth finish for visual and magnetic particle examination.

6.6.2 The manufacturer is to carry out an ultrasonic examination of each forging and is to provide the Surveyor with a signed statement that such inspection has not revealed any significant internal defects.

6.6.3 Rotor forgings for propulsion machinery and for auxiliary turbines exceeding 1100 kW are to be hollow bored for internal examination. The surface of the bore is to have a fine smooth finish and is to be examined by means of an optical instrument of suitable magnification. Where the bore size permits, magnetic particle examination is also to be carried out. These examinations are to be confirmed by the Surveyor. Alternatively, an approved method of ultrasonic examination may be accepted instead of hollow boring. Details of the proposed method of ultrasonic examination are to be submitted for special consideration.

Table 5.6.1 Mechanical properties for acceptance purposes: carbon-manganese steel forgings for turbines - Normalised and tempered

Tensile strength N/mm2	Yield stress N/mm2 minimum	Elongation 5,65 % minimum			Reduction of area % minimum
		A	B	C	A	B	C
400 – 520	200	26	22	18	50	40	35
440 – 560	220	24	21	17	50	40	35
480 – 600	240	22	19	15	45	35	30
520 – 640	260	21	18	14	45	35	30
560 – 680	280	20	17	13	40	30	25
600 – 720	300	18	15	12	40	30	25
Columns A are applicable to longitudinal tests from rotor and spindle forgings. Columns B are applicable to tangential tests from rotor forgings. Columns C are applicable to radial tests from rotor forgings. Intermediate values may be obtained by interpolation.

6.7.1 Thermal stability tests after heat treatment and rough machining of the turbine rotors, referred to in the relevant Rules dealing with design and construction, are to be undertaken in properly constructed furnaces, using accurate and reliable measuring equipment. Each test is to be carried out in accordance with the following recommended procedure:

1. Five bands are to be machined concentric with the axis of rotation. Two of these are to be reference bands and are to be positioned at or near the locations of the bearings. The remaining three bands are to be test bands located one as near as possible to the mid-length, and the other two near each end of the body. Where the length of a rotor is such that five bands cannot be provided, alternative proposals are to be submitted to the Surveyor for his approval.

2. Four positions, 90° apart, are to be stamped A, B, C and D on the coupling end of the rotor.

3. The whole of the body, and as much of the shaft at either end as will include the positions of the glands, is to be enclosed in the furnace. In the case of a rotor having an overhung astern wheel, the astern wheel is also to be enclosed in the furnace during the first test.

4. The rotor is to be rotated at a uniform and very low speed.

5. The deflections at all bands are to be recorded at the A, B, C and D positions. Initial cold readings are to be taken prior to heating.

6. The rotor is to be heated uniformly and slowly. Temperatures are to be recorded continuously at the surface of the rotor and, if practicable, in the bore at the mid-length of the body. In no circumstances is the surface temperature to exceed the temperature at which the rotor was tempered. During heating, the rate of rise of temperature is to be such as to avoid excessive temperature gradients in the rotor.

7. The maximum or holding temperature is to be not less than 28°C above the maximum operating temperature of the rotor. For the purposes of the test, the holding period is to start when the rotor has attained a uniform and specified temperature. The rotor is to be held under the specified temperature conditions until not less than three consecutive hourly readings of deflections show the radial eccentricity to be constant within 0,006 mm on all test bands.

8. The turbine rotor is to be rotated during cooling until the temperature is not more than 100°C. The rate of cooling is to be such as to avoid excessive temperature gradients in the rotor.

9. Final cold readings are to be taken.

Table 5.6.2 Mechanical properties for acceptance purposes: alloy steel forgings for turbines - Quenched and tempered or normalised and tempered

Tensile strength N/mm2 (see Note)	Yield stress N/mm2 minimum Normalised and tempered	Yield stress N/mm2 minimum Quenched and tempered	Elongation on 5,65 % minimum			Reduction of area % minimum
			A	B	C	A	B	C
500 – 650	275	–	22	20	18	50	40	35
550 – 700	300	–	20	18	16	50	40	35
600 – 750	330	410	18	16	14	50	40	35
650 – 800	355	450	17	15	13	50	40	35
700 – 850	385	490	16	14	12	45	35	30
750 – 900	–	530	15	13	11	45	35	30
800 – 950	–	590	14	12	10	45	35	30
850 – 1000	–	640	13	11	9	40	30	25
900 – 1050	–	690	13	11	9	40	30	25
950 – 1100	–	750	12	10	8	40	30	25
1000 – 1150	–	810	12	10	8	40	30	25
Columns A are applicable to longitudinal tests from rotor and spindle forgings. Columns B are applicable to tangential tests from rotor and spindle forgings, and to tangential tests from discs – test position B in Figure 5.6.2 Test positions for turbine disc forgings. Columns C are applicable to radial test from rotor forgings and to tangential tests from discs – test position C in Figure 5.6.2 Test positions for turbine disc forgings. Intermediate values may be obtained by interpolation.

6.7.2 The movements of the axis of the rotor in relation to the reference bands are to be determined from polar plots of the deflection readings. The radial movement of the shaft axis, as determined by the difference between the final hot and the final cold movements, is not to exceed 0,025 mm on any one band. As verification that test equipment and conditions are satisfactory, it is required that similar determinations of differences between initial cold and final cold movements do not exceed 0,025 mm on any one band.

6.7.3 If the results of the test on a rotor fail to meet either or both of the requirements in Ch 5, 6.7 Thermal stability tests 6.7.2, the test may be repeated if requested by the maker and agreed by the Surveyor. In the case of a rotor failing to meet the requirements of a thermal stability test, the rotor is deemed unacceptable. Proposals for the rectification of thermal instability of a rough machined rotor are to be submitted for special consideration.