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The increasingly arduous
conditions encountered in oil well drilling and production operations demand
oil country tubular with improved high yield strength and toughness. One way
of achieving these properties is by quench and temper processing. While Q&T
plants are primarily designed for the production of casing and tubing with
minimum yield strengths of up to 1034 N/mm˛, special property line pipe and
structural hollow sections can also be produced.
The salient features of the process requiring close technical
control are careful selection of steel chemistry, control of austenitising
conditions, efficiency of quenching and control of tempering. All of these
influence the through thickness properties of the product and must be
designed to ensure that variations in physical structure and properties
within the pipe are kept to the absolute minimum.
Process
The layout of the Q & T unit, including heat treatment and
finishing facilities, is shown in Fig.1. Natural gas-fired walking beam
furnaces, capable of taking pipe lengths up to 15.2m (50ft), are used for
heating and the quench system is of the external water quench type. The
austenitising furnace, pressure and temperature control has a multiple
burner arrangement split into eight zones each with air/gas ratio control.
The walking beam system has thirteen holding stations provided with
facilities for constant pipe rotation to ensure uniform heat distribution
within the pipe and the development of a homogeneous austenitic structure
for quenching. Furnace charging and discharging are synchronised and a
series of roller ways is used to impart high speed rotation to the pipe on
discharge into the quench system.
The quench system incorporates an initial air ring to confine the
quench water, and the seven quench rings following this provide the drastic
quenching needed for through hardening. The quench rings have three rows of
uniformly spaced holes angled to allow water to impinge on the pipe along
the direction of travel. The flow rate is 7000 litres per minute at
pressures of up to 10 kN/m˛. After the quench section, a 11 m spray unit
gives a further water delivery of 5000 litres per minute, ensuring no
recalescence in the in the quenched pipe. Reservation is made in the plant
layout for other forms of quench systems should these be required for
specific applications.
The tempering furnace incorporates control facilities similar to
those used for austenitising, but is provided with twenty-one pipe holding
stations to give the longer heating times necessary in tempering.
Heat treatment schedules are determined for each pipe size and the
furnace walking beam mechanisms operated on pre-set cycles to provide
consistent austenitising and tempering conditions throughout the size range.
Control of heat abstraction through the quench unit is varied by adjusting
the rotations speed of the furnace discharge rollers to ensure that the full
cross-section of the pipe is below the Martensite finish (Mf) temperature on
completion of the quench operation. The positioning of the pipe centrally
within the quench unit by adjusting the ring assembly through a worm-gear
mechanism and capping the pipe ends with thin metal discs to prevent water
ingress to the pipe bore, ensures uniform quench conditions along individual
pipe lengths and eliminates distortion problems.
After heat treatment, the pipe are hot sized in a 3-stand sizing
mill. This ensures close compliance with the dimensional tolerances
required, particular for threading. The pipes are then cooled on transfer
racks before straightening in a rotary straightener.
Quality control is maintained by hardness testing and in-line
checking of chemical composition. Each pipe is non-destructively tested by
an ultrasonic rotating probe system, utilising compression and shear wave
ultrasonic techniques. The information so obtained on thickness measurement
and defect detection is electronically processed to provide a permanent
record, and facilities are available for automatic marking of the
longitudinal and quadrant positions of any defects. External surface
imperfections are precisely located by magnetic particle inspection and
removed by grinding, after which all ground areas are ultrasonically checked
for compliance with the thickness specification. The mechanical properties
of the finished pipe are determined as required by specification.
Metallurgical considerations of production control
The optimum combination of strength and toughness is obtained from
tempered martensite. The basic requirement on quenching is transformation of
the pipe thickness to a high percentage of martensite, subject to freedom
from quench cracking arising from internal strain induced by the volume
changes of occurring during transformation. This is effected by limiting the
carbon content to o.34%, controlling austenitising to minimise grain
coarsening, and quenching to give a slight through-thickness gradation in
the degree of martensite transformation. The criterion for quench-hardening
is based on maintaining an across-thickness hardness variation after
tempering of less than 30 Hv.
For API grades, C-75 to P-110, the strength properties are obtained
from steels at 0.28 to 0.34% carbon, by varying the manganese content from
1.0 to 1.6% and the tempering temperature from 560°C to 685°C. The effects
of these variables on the hardness of the steel are illustrated in Fig.2,
which indicates the hardness ranges corresponding to the strength limits of
grades C-75, N-80, C-95 and P-110. In addition to enabling these grades to
be processed from one type of steel, the effect of tempering temperature is
used to control the yield strength within close limits, particularly for
restricted yield strength and high collapse strength grades. For these
special grades it may be necessary to apply cast selection within restricted
composition ranges for tempering at intervals of 20°C.
The major factor limiting the use of carbon steels is pipe
thickness, which influences the attainment of the required martensitic
structure on quenching. The Jominy hardenability characteristics of the
steel are used to define the limiting conditions and determine the level of
alloy additions required to meet the through hardening criterion.
Hardenability concepts are also used for chemical composition control to
ensure adequate hardenability from steel supplied with differing amounts of
residual elements. This control, exercised in relation to chromium and
molybdenum contents, is particularly important in avoiding quench cracking.
For unalloyed carbon steel products, acceptable and upper limits are
specified for chromium and molybdenum and the carbon and manganese contents
adjusted to compensate for additional hardenability induced by residual
element contents above the acceptable limit. Grades with yield strengths
above P-110 require the use of alloy steels in which yield strengthening is
derived mainly from the greater resistance of alloy carbides to tempering.
The alloy additions, used either singly or in combination dependent on
thickness and strength requirements, include the carbide forming elements,
chromium, molybdenum and vanadium. Molybdenum, which is also a ferrite
strengthener and gives the best combination of strength and low temperature
toughness, is generally preferred. For these grades, in which the alloy
additions increase the steel hardenability, the carbon content is limited to
0.32% to minimise any tendency towards quench cracking.
Special types of casing require the pipe ends to be thickened for
enhanced ‘pull-out’ or high pressure sealing properties, and the steel
chemistry and tempering parameters must therefore be selected to take
account of the thickness difference between the ‘upset’ and pipe body. The
steel composition is designed for adequate through hardening at the upset
region, and the tempering treatment controlled to give pipe body strength
properties above the lower quartile of specification.
Mechanical and performance properties
Tensile properties are the essential criteria of process and
metallurgical control for standard API casing grades. The compositions and
tempering treatments used for the C-75 to P-110 grades are given in Table 1.
The wider tempering ranges for the restricted yield strength grades C-75 and
C-95 are required to maintain close control of the yield strength. This is
also essential for high collapse strength grades with a minimum yield
strength of 655 N/mm˛. The Charpy V-notch impact strength properties of the
carbon-manganese steels are dependent to some extent on the steelmaking
deoxidation practice. The transition data obtained are typical of N-80 and
P-110 grades processed from silicon killed steels. For both grades a
Fracture Appearance Transition Temperature (F.A.T.T.) of around –45°C can be
maintained at energy absorption levels of 40-45 J for N-80 grade and 30-35 J
for P-110 grade. Improved notch ductility results from the use of aluminium
treated fine-grained steels and a further enhancement from the use of lower
carbon alloyed steels. Control parameters have been established enabling the
‘F.A.T.T.)’ criterion to be maintained down to – 90°C.
Mechanical tests do not, by themselves, give a realistic assessment
of the performance of high grade pipe under the required operating
conditions, and it may be necessary to apply fracture toughness studies
particularly where other environmental effects such as susceptibility to
sulphide stress corrosion cracking may exist. In casing string design,
control of crack initiation rather than propagation may be more important,
since the complex stress system of high axial and circumferential stress,
combined with mechanical coupling between casing lengths will lead to crack
arrest. Crack-opening displacement and other fracture toughness tests have
shown that quenched and tempered products maintain high-energy ductile
initiation characteristics down to lower temperatures than are obtained for
normalised and tempered material. For quenched and tempered N-80 casing, for
example, brittle fracture initiation will not take place at temperatures
above –100°C, confirming its suitability for service temperatures down to –
80°C. Resistance to sulphide stress corrosion cracking has been studied in
considerable detail. Quenched and tempered steels are accepted as being more
resistant to this type of cracking than normalised and tempered steels, and
the suitability of grades C-75, N-80 and C-95, as produced by quenching and
tempering, has been indicated. With higher ‘bottom hole’ pressures and more
aggressive environments being envisaged with deep well drilling, this
research activity is now receiving considerably more detailed attention. |
