The drill collar is one of the most mechanically demanding components in the entire drill string. Positioned directly above the drill bit, it must simultaneously provide the concentrated weight-on-bit that drives the cutting action, absorb the fatigue loading generated by thousands of rotation cycles per hour, resist the corrosive chemistry of downhole fluids, and maintain dimensional integrity at temperatures and pressures that increase continuously as wells are drilled deeper. As Chinese upstream programmes advance into deep and ultra-deep targets — formations below 5,000 m where temperatures routinely exceed 150 °C and wellbore pressures surpass 120 MPa — the selection and manufacturing qualification of drill collar material becomes a critical engineering decision rather than a commodity procurement exercise.
KH-4145HM is a domestically developed modification of the internationally standardised 4145H chromium-molybdenum alloy steel, optimised specifically for heavyweight drill collar bar production under these severe service conditions. This article provides a complete technical reference covering the KH-4145HM smelting and billet preparation route, the hot rolling process flow and parameter envelope, the heat treatment specification, the dimensional and non-destructive testing acceptance criteria, and the thread and surface finishing requirements — drawing on published process data from Shandong Kehuan Petroleum Equipment Co., Ltd. For operators and procurement engineers who also need to specify the non-magnetic drill collars that accompany standard 4145H collars in every directional and horizontal BHA, many of the quality parameters discussed here — straightness tolerances, NDT acceptance levels, surface finish requirements, and thread make-up specifications — apply equally to both product types.
Why 4145H-Modified Steel for Heavyweight Drill Collars?
The 4145H designation identifies a chromium-manganese-molybdenum alloy steel with a composition centred on approximately 0.4–0.5% carbon, 0.8–1.1% chromium, and 0.15–0.25% molybdenum. The "H" suffix in the API designation indicates that the steel is supplied to hardenability band limits rather than to strict chemical composition limits — a specification approach that ensures consistent through-section mechanical properties regardless of section size. This is essential for drill collar blanks, which are thick-walled heavy sections where hardenability depth directly governs whether the core of the section achieves the required strength and toughness after quench-and-temper heat treatment.
The "KH" prefix and "M" modifier in KH-4145HM reflect Shandong Kehuan's proprietary refinements to the base 4145H composition and process, designed to address specific performance gaps identified in deep well applications: tighter control of sulphur and non-metallic inclusion content for improved fatigue crack resistance, enhanced through-thickness hardenability for the large-diameter sections used in heavyweight drill collars, and optimised tempering response to ensure that the quench-and-temper cycle delivers the target combination of yield strength, impact toughness, and hardness uniformity throughout the full section thickness.
Chromium contributes carbide-forming tendency and hardenability increment. Molybdenum provides additional hardenability and, critically, temper embrittlement resistance — molybdenum suppresses the impurity segregation to prior austenite grain boundaries that causes the toughness loss known as temper brittleness, which is a concern whenever high-strength alloy steels are tempered in the 350–550 °C range. The combination also provides meaningful resistance to the hydrogen embrittlement mechanisms that become increasingly significant as H₂S partial pressures rise in deep sour-gas formations.
Smelting Route: EAF → Ladle Refining → Vacuum Degassing
KH-4145HM Billet Production Route
Stage 1 — Electric Arc Furnace (EAF)
Raw material quality is the first control point in achieving the required steel cleanliness. The EAF charge uses high-purity iron ore (iron purity ≥99.5%), low-sulphur iron concentrate (S ≤0.005%), and high-purity alloying additions (Cr, Mn, Mo each at ≥99.5% purity). Iron ore oxide content is held strictly below 0.05% by mass. The furnace is a 30-tonne DC arc unit operating at 1,600–1,650 °C with temperature control precision of ±5 °C, maintained by precision thermocouple instrumentation and automated process control. Melting cycle duration is 2.5 hours, covering the charging, melting, and holding stages. Furnace lining is constructed from graphite brick and refractory materials to minimise steel-atmosphere contact. Controlled reducing conditions are maintained throughout the melt, and silicon (approximately 0.1%) and aluminium (approximately 0.03%) are added as primary deoxidisers, forming oxide inclusions that are subsequently managed through the downstream refining steps.
Stage 2 — Ladle Refining Furnace (LRF)
After tapping from the EAF, the steel is transferred to the LRF for secondary metallurgical processing. The primary objective at this stage is deep desulphurisation, targeting a final sulphur content at or below 0.005%. High-activity lime (CaO purity ≥95%) is injected at 0.5% of steel mass, distributed uniformly across the melt surface by gas injection. The desulphurisation reaction at 1,600 °C converts dissolved sulphur to calcium sulphide (CaS) slag, which floats and is separated. CaSi alloy (silicon-calcium) is subsequently added at 0.3% of steel mass to stabilise bath basicity and promote further removal of both oxygen and sulphur, improving melt fluidity and cleanliness. LRF temperature is maintained at 1,600–1,650 °C using high-efficiency heat exchange and automated temperature control to prevent thermal fluctuation from disrupting the kinetics of the refining reactions.
Stage 3 — Vacuum Degassing (VD)
The LRF-treated steel is transferred to a dual-chamber vacuum degassing unit. The chamber pressure is reduced to below 0.1 Pa by a high-capacity vacuum pump system. Treatment duration is 45 minutes, during which the pressure differential drives dissolved hydrogen and other volatile gases (oxygen, nitrogen) out of solution, substantially reducing gas porosity and associated non-metallic inclusion content. High-purity aluminium is added at 0.04% of steel mass during this stage as a final deoxidation treatment, forming sub-micron Al₂O₃ inclusions. High-purity argon is used as the degassing medium, and degassing parameters — temperature, duration, and chamber pressure — are optimised to ensure that residual non-metallic inclusions are controlled to sub-micron size, establishing the internal cleanliness baseline that determines the fatigue life of the finished drill collar.
Billet Quality Acceptance: Three Critical Inspection Requirements
Low-Magnification (Macro) Structure Inspection
Billet macro-structure is evaluated by metallographic microscopy following sectioning, polishing, and etching sample preparation. The inspection confirms grain refinement and structural homogeneity, and checks for the following specific defect categories with the indicated acceptance limits. General porosity is rated at 1.0 maximum. Centre porosity is rated at 1.0 maximum. Ingot-type segregation is rated at 1.0 maximum. Point segregation is rated at 0 — meaning no point segregation is acceptable. The billet must be entirely free from gas bubbles, cracks, inclusions, seams, white spots (hydrogen flakes), near-surface cracks, axial intergranular cracks, and laminations at the magnification levels used for low-magnification examination. The preliminary heat treatment applied before macro inspection — 860 °C hold for 20 minutes followed by oil cooling, then 600 °C hold for 90 minutes followed by oil cooling — serves to refine the as-cast grain structure and close any residual casting porosity before assessment.
Non-Metallic Inclusion Rating
Non-metallic inclusions are classified and rated according to the four morphological types defined in GB/T 3077, evaluated by metallographic examination of the billet cross-section following the cut-polish-etch preparation sequence. Type A inclusions (sulphide-type, deformable) are accepted with coarse grade at ≤2.0 and fine grade at ≤2.5. Type B inclusions (aluminate-type, angular, brittle) have no upper limit on coarse grade but fine grade must be ≤2.5. Type C inclusions (silicate-type, glassy) follow the same limits as Type A — coarse ≤2.0, fine ≤2.5. Type D inclusions (globular oxide-type) have no upper limit on coarse grade but fine grade is limited to ≤2.5. The combined application of ladle refining, vacuum degassing, and precise charge calculation is the process mechanism that achieves these inclusion cleanliness levels, with the three-stage smelting route specifically designed to progressively reduce both the volume fraction and the size of inclusion populations in the finished billet.
End-Quench (Jominy) Hardenability Testing
Hardenability is the most critical bulk mechanical property for a thick-section drill collar blank, because it determines whether the entire section cross-section achieves the required hardness — and by extension, the required yield strength and toughness — after the quench-and-temper heat treatment applied to the finished collar. The Jominy end-quench test provides a standardised measure of hardenability depth by recording hardness at increasing distances from the quenched end of a standard test bar. Sample preparation heat treatment for the Jominy test is 860 °C hold for 30 minutes followed by air cooling, then a second 860 °C hold for 30 minutes followed by water quench.
| Measurement Point | Distance from Quenched End | Target HRC (Minimum) | Verified Range | Mean Value |
|---|---|---|---|---|
| Position 1 | 1.5 mm | ≥56 HRC | 56–60 HRC | 56.0 HRC |
| Position 2 | 25 mm | ≥50 HRC | 50–56 HRC | 50.4 HRC |
| Position 3 | 50 mm | ≥40 HRC | 41–52 HRC | 46.3 HRC |
Tested by Rockwell hardness method (HRC scale). Measurements taken at three standardised positions: 1.5 mm, 25 mm, and 50 mm from the quenched end face.
The retention of 46.3 HRC mean hardness at 50 mm from the quenched end is particularly significant for large-diameter drill collar blanks, confirming that the Cr-Mo alloy chemistry of KH-4145HM sustains adequate hardenability through a cross-section dimension far exceeding 50 mm, ensuring that even the geometric centre of a thick-wall collar blank will transform to martensite (rather than bainite or ferrite-pearlite) during production quenching.
Hot Rolling Process: From Billet to Finished Tube
Furnace Heating — Single-Accumulator Annular Furnace
Billets enter a single-accumulator annular heating furnace set to achieve uniform billet temperature of 1,200 °C before piercing. The furnace is divided into six functional temperature zones with the following control setpoints, progressing from the furnace entry to the discharge port.
| Zone | Preheat | Heating Zone 1 | Heating Zone 2 | Heating Zone 3 | Soaking Zone 1 | Soaking Zone 2 | Exit |
|---|---|---|---|---|---|---|---|
| Temperature (°C) | 550–570 | 750–1,000 | 1,000–1,080 | 1,100–1,180 | 1,200–1,220 | 1,220–1,250 | 1,260 |
The progressive zone temperature profile ensures that billets do not experience thermal shock from rapid surface heating while the core remains cold — a critical requirement for a thick-section Cr-Mo alloy steel that could develop thermal stress cracking if heated too aggressively in the early furnace zones. Holding in the two soaking zones at 1,200–1,250 °C ensures that the full billet cross-section reaches a uniform austenite temperature before the billet exits the furnace, eliminating temperature gradients that would otherwise produce uneven deformation and non-uniform wall thickness during piercing.
Piercing — Two-Roll Skew Rolling Mill
The heated billet is pierced on a two-roll skew rolling mill to form a hollow shell (毛管). The critical piercing parameters are cross-section reduction rate at 10%, draft of 28 mm, guide plate ellipticity coefficient of 1.05, tip front reduction rate of 6%, and extension coefficient of 2.1. The skew rolling geometry creates a rotating, advancing deformation zone around the piercing plug that produces the hollow shell with a spiral surface finish, which is then removed by the subsequent rolling stages. Maintaining the correct ellipticity coefficient in the guide plates is essential for controlling oval out-of-roundness in the pierced shell before it enters the mandrel mill.
Rolling — ASSEL Dual-Mandrel Three-Roll Mill
The ASSEL mill is the productivity and quality heart of this production route. Unlike conventional elongator mill designs that use a single mandrel, the ASSEL dual-mandrel configuration allows the second mandrel to be prepared and loaded while the first mandrel is in operation, substantially improving mill utilisation. The three-roll arrangement provides symmetric radial reduction with no bending moment imbalance, which improves wall thickness uniformity compared to two-roll alternatives. The key ASSEL mill parameters for KH-4145HM production are cross-section compression ratio of 30%, shell reducing ratio of 26.5%, and both the rolling angle and the advance angle controlled at 6°. The advance angle directly governs the axial feed rate relative to the circumferential rolling speed; holding it at 6° optimises the deformation efficiency while preventing the transverse wave defect (spiral wall thickness variation) that can occur when the advance angle is set too high.
Sizing — 12-Stand Micro-Tension Reducing Mill
After the ASSEL mill, the shell passes through a 12-stand micro-tension reducing mill with each stand applying 2% diameter reduction per pass. The micro-tension between stands prevents the tube from thickening in the bore (a phenomenon known as wall thickening in sizing) while delivering the precise final outer diameter required for the subsequent machining allowance. Sizing temperature is controlled at 980–1,100 °C; the temperature drops progressively from 850 to 930 °C after sizing as the tube exits onto the cooling bed. This exit temperature window is critical: it is above the Ms (martensite start) temperature, which means the tube structure is still austenitic and will transform during subsequent controlled cooling rather than quenching to martensite in an uncontrolled manner on the rolling line.
Straightening and Preliminary NDT
After air cooling on the cooling bed, the tube passes through a straightening machine for initial straightness correction, eliminating the bowing and camber introduced by non-uniform cooling across the tube length. At this stage, the tube is then subjected to both magnetic flux leakage (MFL) testing and ultrasonic testing (UT) to identify any surface or sub-surface defects introduced during hot rolling. Tubes passing this preliminary NDT proceed to end flaring (expanding the bore at both ends to accommodate the subsequent thread machining operations) and the full heat treatment sequence.
Post-Rolling Heat Treatment
The heat treatment sequence applied to the rolled and sized tube before final machining consists of three stages. Normalising is performed at 860 °C with a 30-minute hold followed by air cooling, which homogenises the microstructure and eliminates the banded structure inherited from hot working. Quenching follows immediately: the tube is reheated to (860 ± 20) °C, held for 30 minutes, then oil-quenched to transform the austenite to martensite throughout the section cross-section. The ±20 °C tolerance band on the quench temperature is a controlled process parameter: insufficient temperature means incomplete austenitisation and loss of hardenability, while excessive temperature promotes austenite grain growth and corresponding reduction in toughness. Tempering at (640 ± 20) °C with a 60-minute hold converts the brittle as-quenched martensite to tempered martensite with the target combination of yield strength, hardness, and impact energy. The 640 °C tempering temperature places the steel in the high-temperature tempering regime well above the temper embrittlement range (350–550 °C), producing stable mechanical properties with good toughness. After heat treatment, a second pass through the 35 MN vertical hydraulic straightening press restores dimensional straightness, and a final UT inspection confirms that the heat treatment process has not introduced new internal defects.
Dimensional Tolerances and NDT Acceptance Criteria
Non-Destructive Testing Requirements
The combination of MFL and UT inspections applied at both the pre-heat-treatment and post-heat-treatment stages provides a comprehensive defect detection record. MFL is particularly sensitive to longitudinal surface defects introduced during hot rolling; UT is sensitive to both surface and sub-surface defects including hydrogen flakes, internal laminations, and heat treatment cracks. Running both inspections in sequence ensures that no category of defect can pass undetected through the manufacturing process.
Thread Machining, Surface Treatment, and Final Finishing
After the tube body has passed all hot rolling, heat treatment, and NDT qualification requirements, the final manufacturing stage covers the thread machining and surface treatment specifications that determine the collar's make-up performance and corrosion resistance in service.
Pre-Threading Heat Treatment
Before threading begins, the full cross-section of the collar blank must receive a complete normalise-quench-temper cycle if this has not already been applied as part of the tube body treatment. The sequence is normalising at 860 °C for 30 minutes (air cool), followed by quenching at 860 °C for 30 minutes (water quench), followed by tempering at 640 °C for 60 minutes. This ensures that the microstructure in the thread zone has the correct tempered martensite condition before machining cuts through the outer surface layers.
Thread Surface Treatment
All internal and external thread surfaces, including the shoulder faces, must be treated with either copper plating or phosphating to improve corrosion resistance and galling resistance during make-up. The copper plating route is preferred where it can be applied, because the copper film provides more consistent lubrication during repeated make-up and break-out operations compared to phosphate conversion coatings. Thread connection designs incorporate stress relief grooves (应力分散槽) at the thread roots to redistribute the peak stress concentration that would otherwise develop at the last engaged thread, the primary fatigue initiation site in drill collar connections. Following thread inspection and acceptance, cold thread rolling can be applied on customer request to further work-harden the thread surface, improving surface compressive residual stress and thread form accuracy simultaneously.
Thread Surface Roughness Requirements
Thread surface finish is a direct determinant of both the make-up torque behaviour and the wear rate over successive connection cycles. The KH-4145HM specification imposes three distinct roughness limits depending on surface function. The internal and external thread shoulder faces — the primary torque-transmitting surfaces that develop the thread pre-load — must achieve Ra ≤3.2 µm. The thread working flanks, which carry the axial load during make-up, are specified at Ra ≤6.3 µm. The thread crest surfaces are the least functionally critical and are permitted a more relaxed Ra ≤12.5 µm. These requirements are enforced under the SY/5144 standard and are consistent with the surface finish requirements in API Spec 7 for rotary drilling drill string components.
Surface Defect Prohibitions
The tube body outer surface must be free from cracks, folds, pits, and seams. The inner bore must be free from shoulders and spiral grooves. If any surface defect is found, it may be removed by grinding, provided that the ground area blends smoothly into the surrounding surface with a radius transition (no sharp geometric discontinuities), and the grinding depth does not exceed the maximum specified in the applicable standard. No grinding is permitted within 500 mm of any thread shoulder face — this exclusion zone protects the dimensional integrity of the critical connection region. Weld repair is categorically prohibited on any drill collar surface; weld deposits would introduce microstructural discontinuities and residual stress concentrations that would substantially degrade fatigue performance in the affected zone.
Applicable Standards and Compliance Framework
KH-4145HM heavyweight drill collars are manufactured and inspected against the Chinese national standard SY/5144, which defines the dimensional, mechanical, chemical, and inspection requirements for oil country drill collars. The specification also references and incorporates relevant technical content from API Spec 7 (Specification for Rotary Drill Stem Elements), which is the global reference standard for drill string components and represents the qualification benchmark for products entering international markets. Non-destructive testing is conducted per ASTM E309 (MFL) and ASTM E213 (UT), which are internationally recognised pipe inspection standards with well-defined reference calibration requirements. Inclusion rating is performed per GB/T 3077. The multi-standard compliance framework positions KH-4145HM collars for use in both domestic Chinese drilling programmes and internationally tendered projects where API certification is required.
Frequently Asked Questions
Q: What is the significance of the ASSEL dual-mandrel three-roll mill in drill collar tube production, and why is it preferred over conventional elongator mill configurations?
The ASSEL mill's three-roll geometry provides symmetric radial reduction around the tube circumference with no net bending moment, which produces significantly better wall thickness uniformity than two-roll elongator designs. The three rolls are arranged at 120° spacing around the tube axis, each applying equal deformation forces radially inward. The dual-mandrel feature improves production efficiency by allowing one mandrel to be prepared and loaded while the other is working, eliminating the idle time that would otherwise occur in a single-mandrel design. For drill collar tubes, where precise and uniform wall thickness is critical for both the dimensional acceptance criteria and the mechanical performance in fatigue loading, the ASSEL mill offers advantages in wall thickness consistency that directly translate to higher acceptance rates in the finished product inspection. The rolling angle and advance angle — both controlled at 6° in the KH-4145HM process — are the key process parameters that balance axial advance rate against circumferential deformation efficiency, controlling the potential for transverse wave defects in the tube wall.
Q: Why is the tempering temperature for KH-4145HM set at 640 °C rather than a lower temperature, and what would happen to mechanical properties if tempering were performed at a lower temperature?
The 640 °C tempering temperature is selected to sit well above the temper embrittlement range of 350–550 °C for Cr-Mo alloy steels. In this temperature range, phosphorus and other tramp elements segregate to prior austenite grain boundaries during slow cooling from the tempering temperature, causing a dramatic reduction in impact toughness (measured by Charpy impact test) without a significant change in hardness or tensile strength. This condition — called temper embrittlement — is particularly dangerous because it is not visible in routine hardness testing, but makes the steel susceptible to brittle fracture under shock loading. At 640 °C, which is within the high-temperature tempering range, the dislocation density in the martensitic microstructure is substantially reduced, carbide particles coarsen and stabilise, and the result is a tempered martensite structure with the target balance of strength and toughness. Tempering at a lower temperature — say 400–500 °C — would retain higher hardness and tensile strength but would sacrifice impact energy severely and risk temper embrittlement during the cooling cycle, which is unacceptable for a fatigue-loaded drill collar application.
Q: Why does the billet specification require Jominy hardenability of ≥40 HRC at 50 mm from the quenched end, and what section size does this correspond to in a finished drill collar?
The Jominy end-quench test standardises the cooling rate at each distance from the quenched end: 50 mm from the quenched end corresponds to a cooling rate roughly equivalent to the geometric centre of a bar approximately 100–120 mm in diameter under ideal quench conditions. For a drill collar with an outer diameter of 172 mm (a common size for directional BHA applications) and a wall thickness of approximately 60–70 mm, the centre of the wall section experiences a cooling rate during production quenching that is slower than the geometric ideal, meaning the actual hardenability demand is even higher than what the 50 mm Jominy point represents. The ≥40 HRC minimum at 50 mm — achieved at 46.3 HRC mean in the KH-4145HM process — confirms that the steel's alloy content is sufficient to sustain hardenability deep into large sections, ensuring that the full wall thickness transforms to martensite during quenching rather than partially transforming to the softer bainitic or ferritic-pearlitic microstructures that would result in inadequate and non-uniform mechanical properties across the collar cross-section.
Q: What is the functional difference between the MFL and UT inspection methods applied to drill collar tubes, and why are both required?
Magnetic flux leakage and ultrasonic testing detect fundamentally different defect categories by different physical mechanisms, and each has sensitivity in areas where the other is limited. MFL (per ASTM E309) works by magnetising the tube and detecting the leakage field created when a surface or near-surface defect disrupts the magnetic flux lines. It is highly sensitive to longitudinal surface-breaking defects — seams, laps, and rolling defects that run parallel or near-parallel to the tube axis — and can be applied at high throughput without surface contact. However, MFL sensitivity decreases rapidly for defects that are deeper than approximately one wall thickness below the surface, and it has limited sensitivity to transverse surface defects. UT (per ASTM E213) works by coupling ultrasonic pulse-echo energy into the tube wall and detecting reflections from internal discontinuities. It is sensitive to both surface and sub-surface volumetric defects — laminations, hydrogen flakes, large inclusion clusters, and heat treatment cracks — that MFL would miss because they create no surface flux leakage. Running both inspections in sequence, at both pre-heat-treatment and post-heat-treatment stages, ensures that no defect category can pass undetected through the full manufacturing sequence.
Drill Collar Components for Deep Well BHA Applications
The technical demands that drive the KH-4145HM manufacturing specification — hardenability depth in large sections, inclusion cleanliness for fatigue resistance, dimensional precision for make-up reliability, and thread surface finish for repeated connection cycles — are the same performance dimensions that govern the specification of every drill collar type in a deep well BHA, including the non-magnetic drill collars that every directional and horizontal well programme also requires. Standard 4145H heavyweight drill collars and non-magnetic drill collars work in the same drill string, subject to the same mechanical loading environment, and must meet equivalent dimensional and surface quality standards. If you are specifying non-magnetic drill collars — Cr-Mn-N austenitic grades for MWD/LWD survey instrument isolation — for programmes that also use KH-4145HM or 4145H standard collars, we invite you to review our product range and discuss grade selection, dimensional specifications, and material certification requirements. Visit gaslinepipe.com or contact our technical team to request product data sheets and material certification samples.
