Section
1 General requirements
1.1 Scope
1.1.1 This Section
gives the general requirements for steel forgings intended for use
in the construction of ships, other marine structures, machinery,
boilers, pressure vessels and piping systems. These requirements are
also applicable to rolled slabs and billets used as a substitute for
forgings and to rolled bars used for the manufacture (by machining
operations only) of shafts, bolts, studs and other components of similar
shape.
1.1.3 As an alternative
to Ch 5, 1.1 Scope 1.1.2, steel forgings which
comply with National or proprietary specifications may be accepted
provided that these specifications give reasonable equivalence to
the requirements of this Chapter or alternatively are approved for
a specific application. Generally, survey and certification are to
be carried out in accordance with the requirements of Ch 1 General Requirements.
1.1.4 Where small
forgings are produced in large quantities, or where forgings of the
same type are produced in regular quantities, alternative survey procedures
in accordance with Ch 1, 2.4 Materials Quality Scheme may be
adopted.
1.2 Manufacture
1.2.1 Forgings are to be made at works which have been approved by Clasifications
Register (hereinafter referred to as LR). The steel used is to be manufactured in
accordance with the requirements of the Rules for the Manufacture, Testing and
Certification of Materials,
Ch 3, 1.4 Manufacture. Where the steel is produced at a separate works to the forging, the
steel manufacturer is also to be approved by LR.
1.2.2 When forgings are made directly from ingots, or from blooms or billets
forged from ingots, the ingots are to be cast in chill moulds with the larger
cross-section uppermost and with efficient feeder heads.
1.2.3 Adequate top and bottom discards are to be made to ensure freedom from
piping and harmful segregations in the finished forgings.
1.2.4 The forgings are to be gradually and uniformly hot worked and are to be
formed as closely as possible to the finished shape and size. The plastic deformation is
to be such as to ensure soundness, uniformity of structure and satisfactory mechanical
properties after heat treatment.
1.2.5 For certain components, such as crankshafts, where grain flow is required
in the most favourable direction having regard to the mode of stressing in service, the
proposed method of manufacture may require special approval by LR. In such cases, tests
may be required to demonstrate that a satisfactory structure and grain flow are
obtained.
1.2.6 The reduction ratio (reduction of area expressed as a ratio) is to be
calculated with reference to the average cross-sectional area of the ingot or
continuously cast material, where appropriate. Where an ingot is initially upset, this
reference area may be taken as the average cross-sectional area after this
operation.
1.2.7 For components forged directly from ingots or from forged blooms or
billets, and in which the fibre deformation is mainly longitudinal, the reduction ratio
is not to be less than 3:1.
1.2.8 For forgings made from rolled billets, or where fibre deformation has taken
place in more than one direction, the reduction ratio is not to be less than 4:1.
1.2.9 Where rolled bars are used as a substitute for forgings and the
requirements of Ch 5, 1.2 Manufacture 1.2.2 are not complied with, the reduction ratio is to be
not less than 6:1 unless otherwise accepted by the LR Surveyor.
1.2.11 Disc type forgings, such as gear wheels, are to be made by upsetting, and
the thickness of any part of the disc is to be not more than one-half of the length of
the billet from which it was formed, provided that this billet has received an initial
forging reduction of not less than 1,5:1. Where the piece used has been cut directly
from an ingot, or where the billet has received an initial reduction of less than 1,5:1,
the thickness of any part of the disc is to be not more than one-third of the length of
the original piece.
1.2.12 Rings and other types of hollow forgings are to be made from pieces cut from
ingots or billets and which have been suitably punched, bored or trepanned prior to
expanding or hollow forging. Alternatively, pieces from hollow cast ingots may be used.
The wall thickness of the forging is to be not more than one-half of the thickness of
the prepared hollow piece from which it was formed. Where this is not practicable, the
forging procedure is to be such as to ensure that adequate work is given to the piece
prior to punching, etc. This may be either longitudinal or upset working of not less
than 2:1.
1.2.13 The shaping of forgings or rolled slabs and billets by flame cutting,
scarfing or arc-air gouging is to be undertaken in accordance with recognised good
practice and, unless otherwise approved, is to be carried out before the final heat
treatment. Preheating is to be employed where necessitated by the composition and/or
thickness of the steel. For certain components, subsequent machining of all flame cut
surfaces may be required, see
Ch 5, 4.2 Manufacture 4.2.4.
1.2.14 Where two or more forgings are joined by welding to form a composite
component, details of the proposed welding procedure are to be submitted for approval.
Welding approval procedure tests are to be required in accordance with Rules for the
Manufacture, Testing and Certification of Materials, Ch 12 Welding Qualifications.
1.2.15 It is recognised that not every forged component type that may be subject to
classification is included in this Chapter. In such cases where the particular component
or type is not included, appropriate National/International Standards may be applied, to
determine the appropriate testing regime and defect acceptance criteria.
1.3 Quality
1.3.1 All forgings
are to be free from surface or internal defects which would be prejudicial
to their proper application in service.
1.4 Chemical composition
1.4.1 All forgings
are to be made from killed steels, and the chemical composition of
ladle samples is to comply with the requirements detailed in subsequent
Sections in this Chapter. Where general overall limits are specified,
the chemical composition selected is to be appropriate for the type
of steel, dimensions and required mechanical properties of the forgings
being manufactured.
1.4.2 Except where
otherwise specified, suitable grain refining elements such as aluminium,
niobium or vanadium may be used at the discretion of the manufacturer.
The content of such elements is to be reported in the ladle analysis.
1.5 Heat treatment
1.5.2 Heat treatment is to be carried out in a properly constructed furnace which
is efficiently maintained and has adequate means of temperature control. The furnace
dimensions are to be such as to allow all the steel forgings to be uniformly heated to
the necessary temperature. In the case of very large forgings, alternative methods of
heat treatment will be specially considered. Sufficient thermocouples are to be
connected to the steel forging(s) in the furnace to show that the temperature is
adequately uniform and the temperatures are to be recorded throughout the heat
treatment. Copies of these records are to be presented to the Surveyor together with a
sketch showing the positions at which the temperature measurements were carried out. The
records are to identify the furnace that was used and give details of the steel-making
heat, the heat treatment temperature, time at temperature and the date. The Surveyor is
to examine the charts and confirm the details on the certificate.
1.5.3 As an alternative procedure to the connection of thermocouples as required
by Ch 5, 1.5 Heat treatment 1.5.2, temperature uniformity surveys of
the heat treatment furnace may be accepted subject to approval by LR, Materials and NDE
department. An initial temperature uniformity survey in the furnace fully loaded
condition is required to be carried out in accordance with recognised standards (e.g.
ASTM A991/A991M). Regular temperature uniformity surveys are then subsequently required
at a frequency of at least once per year.
1.5.4 Where forgings are to be quenched and tempered and cannot be hot worked
close to size and shape, they are to be suitably rough machined or flame cut prior to
being subjected to this treatment.
1.5.5 If for any reason a forging is subsequently heated for further hot working,
the forging is to be reheat treated.
1.5.6 If any straightening operation is performed after the final heat treatment,
consideration should be given to a subsequent stress relieving heat treatment in order
to avoid the possibility of harmful residual stresses. The manufacturer shall have
strict control of the stress relieving heat treatment temperature in order to avoid any
detrimental effects on the final heat treatment and the resultant microstructure and
mechanical properties of the forging.
1.5.7 Where it is intended to surface harden forgings, full details of the
proposed procedure and specification are to be submitted for approval. For the purposes
of this approval, the manufacturer will be required to demonstrate by tests that the
proposed procedure gives a uniform surface layer of the required hardness and depth and
that it does not impair the soundness and properties of the steel.
1.5.8 Where induction hardening or nitriding is to be carried out after
machining, forgings are to be heat treated at an appropriate stage to a condition
suitable for this subsequent surface hardening.
1.5.9 Where carburising is to be carried out after machining, forgings are to be
heat treated at an appropriate stage (generally either by full annealing or by
normalising and tempering) to a condition suitable for subsequent machining and
carburising.
1.5.10 The forge is to maintain records of heat treatment identifying the furnace
used, furnace charge, thermocouple location, date, temperature and time at temperature.
The records are to be presented to the Surveyor on request.
1.6 Test material
1.6.1 Test material, sufficient for the required tests and for possible re-test
purposes, is to be provided with a cross-sectional area of not less than that part of
the forging which it represents. This test material is to be integral with each forging,
except in the case of small forgings which are batch tested, see
Ch 5, 1.6 Test material 1.6.4.
1.6.2 Where a forging is subsequently divided into a number of components, all of
which are heat treated together in the same furnace, for test purposes this may be
regarded as one forging and the number of tests required is to be related to the total
length and mass of the original multiple forging, see
Ch 5, 2.4 Mechanical tests 2.4.2.
1.6.4 Where a number of small forgings of about the same size are made from one
cast and heat treated together in the same furnace, batch testing procedures may be
adopted using one of the forgings for test purposes, or alternatively using separately
forged test samples. These test samples are to have a forging reduction similar to that
used for the forgings which they represent. They are to be properly identified and heat
treated together with the forgings. Unless otherwise agreed the maximum allowable batch
size is to be as follows:
- Normalised forgings with mass up to 1000 kg each may be batch tested. A batch is
to consist of forgings of similar shape and dimensions, made from the same
steel-making heat, heat treated together and with a total mass not exceeding 6
tonnes.
- Quenched and tempered forgings with mass up to 500 kg each may be batch tested. A
batch is to consist of forgings of similar shape and dimensions, made from the
same steel-making heat, heat treated together in the same furnace and with a total
mass not exceeding 3 tonnes.
- A batch testing procedure may also be used for hot rolled bars, see Ch 5, 3.4 Mechanical tests 3.4.3.
1.6.5 When required by the conditions of approval for surface hardened forgings
(see
Ch 5, 1.5 Heat treatment 1.5.7) additional test
samples are to be processed at the same time as the forgings which they represent. These
test samples are subsequently to be sectioned in order to determine the hardness, shape
and depth of the locally hardened zone and which of them are to comply with the
requirements of the approved specification.
1.7 Mechanical tests
1.7.2 Test specimens
are normally to be cut with their axes either mainly parallel (longitudinal
test) or mainly tangential (tangential test) to the principal axial
direction of each product.
1.7.3 The test specimens shall be positioned as follows:
-
For forgings with a thickness, t, or diameter D up to a
maximum 50 mm, the longitudinal axis of the test specimen is to be located at a
distance of t/2 or D/2 below the heat-treated surfaces.
-
For forgings with a thickness, t, or diameter D greater
than 50 mm, the longitudinal axis of the test specimen is to be located at a
distance of t/4 or D/4 (mid-radius) or 80 mm, whichever is less,
below any heat-treated surface. Test specimen is to be located with its
longitudinal axis at a distance from any heat-treated surface as shown in Figure 5.1.1 Position of the test specimen.
-
For ring and disc forgings (note that the test specimen locations for
these shaped forgings may be different to elongated or free from forgings),
tangential samples shall be taken at t/2 for thickness ≤ 25 mm and 12,5 mm
below the surface for thickness greater than 25 mm, in both the vertical and
horizontal directions as shown in Figure 5.1.1 Position of the test specimen.
Where achievable, for thickness greater than 25 mm, no part of the test
material shall be closer than 12,5 mm to any heat-treated surface, as shown in Figure 5.1.1 Position of the test specimen.
Figure 5.1.1 Position of the test specimen
1.7.4 Where the manufacturer can demonstrate that a proposed testing location or orientation
is more representative of the required mechanical properties of a component, this may be
agreed with LR. In such cases, the heat treatment process, a proposed testing location
or orientation, and technical justification, shall be submitted to LR for approval.
1.7.5 Tensile test
specimens are to be machined to the dimensions detailed in Ch 2 Testing Procedures for Metallic Materials. Where this is precluded by the dimensions
of the forging, the test specimen is to be of the largest practicable
cross-sectional area.
1.7.8 Hardness
tests, preferably of the Brinell type, are to be carried out when
specified in subsequent Sections in this Chapter.
1.8 Visual and Non-Destructive Examination
1.8.1 All forgings should be subjected to a 100 per cent visual examination of
all accessible surfaces by the manufacturer and the results of these examinations made
available to the Surveyor. Where applicable, this visual examination is to include the
examination of internal surfaces and bores. Unless otherwise agreed, the verification of
dimensions is the responsibility of the manufacturer.
1.8.2 Forgings
are to be examined in the condition for final delivery. Surfaces are
to be clean and free from dirt, grease, paint, etc. Black forgings
are to be suitably descaled by either shotblasting or flame descaling
methods.
1.8.3 All forgings
are to be free of cracks, crack-like indications, laps, seams, folds,
or other injurious indications. At the request of the Surveyor, additional
magnetic particle, dye penetrant and ultrasonic testing may be required
for a more detailed evaluation of surface irregularities.
1.8.4 When specified in subsequent Sections in this Chapter, or by an approved
procedure for welding composite components, see
Ch 5, 1.2 Manufacture 1.2.14, appropriate Non-Destructive Examination is also
to be carried out before acceptance. All tests are to be carried out in accordance with
the requirements of Ch 1, 5 Non-destructive examination. In addition to the information listed in Ch 1, 5.6 Non-destructive examination reports, test results of
surface inspections are to include the following items:
- Testing method and testing details, including procedure number:
- For liquid penetrant testing, the penetrant system used and viewing
conditions (as appropriate to the penetrant technique and media used);
- For magnetic particle testing, method of magnetising, test media, magnetic
field strength, magnetic flux indicators (where appropriate) and viewing
conditions (as appropriate to the magnetising technique and media used).
- Details of weld repair including sketch (where applicable).
1.8.5 Magnetic particle and liquid penetrant testing is to be carried out when
the forgings are in the finished machined condition, see also
Ch 1, 2.3 Materials Survey Scheme 2.3.5. For magnetic particle testing, attention is to
be paid to the contact between the forging and the clamping devices of stationary
magnetisation benches in order to avoid local overheating or burning damage on its
surface. Prods are not permitted on finished machined items. The surface inspection is
to be carried out before the shrink fitting, where applicable. Unless otherwise agreed,
these tests are to be carried out in the presence of the Surveyor. Other surface
inspection methods, e.g. eddy current testing, may be required by LR as a supplementary
method, e.g. for confirming the presence of indications, or for detecting the presence
of undocumented weld repairs.
1.8.6 The following definitions apply to indications associated with magnetic
particle and liquid penetrant inspection:
-
Linear indication. An indication with a largest dimension, l, three
or more times its smallest dimension, w (i.e. l ≥ 3w).
-
Nonlinear indication. An indication with a largest dimension less than
three times its smallest dimension (i.e. l < 3w).
-
Aligned indication.
- Non-linear indications form an alignment when the distance between
indications is less than 2 mm and at least three indications are aligned. An
alignment of indications is considered to be a unique indication and its
length is equal to the overall length of the alignment.
- Linear indications form an alignment when the distance between two
indications is smaller than the length of the longest indication.
-
Open indication. An indication visible after removal of the magnetic
particles or that can be detected by the use of liquid penetrant testing.
-
Non-open indication. An indication that is not visually detectable after
removal of the magnetic particles or that cannot be detected by the use of liquid
penetrant testing.
-
Relevant indication. An indication that is caused by a condition or type of
discontinuity that requires evaluation. Only indications which have any dimension
greater than 1,5 mm are to be considered relevant for the categorisation of
indications.
1.8.7 Acceptance
standards for defects found by visual or non destructive examinations
are to be in accordance with any specific requirements of the approved
plan, and with equivalence to any additional requirements of this
Chapter. In all cases they are to be to the satisfaction of the Surveyor.
1.8.8 Where required, ultrasonic examination is to be carried out after the
forgings have been machined to a condition suitable for this type of examination and
after the final heat treatment, but prior to the drilling of oil bores, prior to surface
hardening and the machining of bolt threads. Both radial and axial scanning are to be
carried out where appropriate for the shape and the dimensions of the forgings being
examined. Scanning is to take into account near surface examination. Unless otherwise
agreed, examinations are to be carried out by the manufacturer, although Surveyors may
request to be present in order to verify that the examination is being carried out in
accordance with the agreed procedure.
1.8.9 If the forging
is supplied in the black condition for machining at a separate works,
the manufacturer is to ensure that a suitable ultrasonic examination
is carried out to verify the internal quality of the forging.
1.8.11 In the
event of any forging proving defective during subsequent machining
or testing, it is to be rejected notwithstanding any previous certification.
1.8.12 Fillet radii should be examined using 45°, 60° or 70° probes, primarily to
determine the presence of any cracks within the radiused areas, and as an additional
scan to confirm any indications that may have been detected with 0° probe(s) within this
area.
1.8.14 Construction of DAC curves for straight beam or normal probes are to be
performed using reference blocks containing suitably sized Flat Bottom Holes (FBH)
spaced over the inspection thickness. Reference blocks are to be manufactured from
similar material, with a surface condition similar to that being inspected. Where
necessary, allowances are to be made for attenuation losses by performing a transfer
correction and adjusting the DAC curve as required. The applied transfer correction
(measured in decibels (dB)) is to become the new reference sensitivity, against which
indications are evaluated, according to the appropriate table contained in this Chapter,
see
Ch 5, 3.5 Non-Destructive Examination 3.5.6 and Table 5.4.5 Ultrasonic acceptance criteria for
crankshafts: DGS Method – Normal probes.
1.8.16 Where angle probes are used, the reference sensitivity is to be established
by using either DAC or DGS methods. DAC sensitivity shall be established from 3 mm side
drilled holes and the DGS method shall be based on appropriately sized flat bottom
holes. Reference blocks are not required for the DGS method unless it is required to
check the accuracy of a particular DGS diagram.
1.8.17 Advanced NDE methods, as described in Ch 1, 5.11 Advanced NDE methods, may be applied to
forgings, as appropriate to the material type, thickness, complexity and geometry.
Acceptance and rejection criteria levels are to be as per the applicable Section of this
Chapter.
1.8.18 Where such standards are used or referenced as a basis for acceptance and rejection
criteria, the quality level is to be equivalent to the allowable criteria stated in the
appropriate tables within this Chapter. The quality levels would normally be the
highest, or most stringent, to provide reasonable equivalence.
1.9 Rectification of defects
1.9.1 Small surface
imperfections may be removed by grinding or by chipping and grinding.
Complete elimination of these imperfections is to be proved by magnetic
particle or dye penetrant examination (as appropriate to the material).
At the discretion of the Surveyor, the resulting shallow grooves or
depressions can be accepted, provided that they are blended by grinding.
1.9.2 Repairs by
welding are not generally permitted, but special consideration will
be given to such repairs where they are of a minor nature and in areas
of low working stresses. In such cases, full details of the proposed
repair and subsequent inspection procedures are to be submitted for
review by the Surveyors prior to the commencement of the proposed
rectification. A report and/or sketch detailing the extent and location
of all repairs, together with details of the post-weld heat treatment
and non-destructive examination are to be provided for record purposes
and are to be attached to the certificate.
1.9.3 Repair welding is not permitted for forgings subject to torsional fatigue,
such as crankshafts and propeller shafts.
1.9.4 Where fabrication
welding is involved, see
Ch 5, 1.2 Manufacture 1.2.14, any repair of defects is to be carried out in accordance
with the approved welding procedure.
1.9.5 The forging
manufacturer is to maintain records of repairs and subsequent inspections
traceable to each forging. The records are to be presented to the
Surveyor on request.
1.9.6 Non-open
indications evaluated as segregation are acceptable.
1.10 Identification
1.10.1 The manufacturer is to adopt a system of identification, as per the requirements of
Ch 1, 4.8 Identification of materials, which will
enable all finished forgings to be traced to the original cast, forging process and heat
treatment batch, and the Surveyor is to be given full facilities to trace the forgings
when required.
1.10.2 Forgings
are to be clearly marked by the manufacturer in accordance with the
requirements of Ch 1 General Requirements. The following
details are to be shown on all forgings which have been accepted:
-
Manufacturer’s name or trademark, steel quality, materials grade,
identification number, cast number or other marking which will enable the full
history of the forging to be traced.
-
LR or Clasifications Register
and the abbreviated name of LR's local office.
-
Personal stamp of
Surveyor responsible for inspection.
-
Test pressure, where
applicable.
-
Date of final inspection.
1.10.3 Modified
arrangements for the identification of small forgings manufactured
in large numbers, as with closed-die forgings may be agreed with the
Surveyor.
1.11 Certification of materials
1.11.2 The manufacturer
is to provide the Surveyor with a written statement giving the following
particulars for each forging or batch of forgings which has been accepted:
-
Purchaser's name and
order number.
-
Description of forgings
and steel quality.
-
Identification number.
-
Steel-making process,
cast number and chemical analysis of ladle samples.
-
General details of heat treatment, including temperature and holding
times.
-
Results of mechanical
tests.
-
Test pressure, where
applicable.
-
Results of non-destructive tests, where applicable.
1.11.3 As a minimum,
the chemical composition of ladle samples is to include the content
of all the elements detailed in the specific requirements.
1.11.5 When steel
is not produced at the works at which it is forged, a certificate
is to be supplied by the steelmaker stating the process of manufacture,
cast number and the chemical composition of ladle samples. The works
at which the steel was produced is to have been approved by LR, see
Ch 5, 1.4 Chemical composition 1.4.3.
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