Tungsten – Carbides Hardfacing of the Drill – Stem Components

V. Ulmanu, G. Draghici, G. Zecheru, M. Minescu“Petroleum – Gas” University of Ploiesti, B-dul Bucuresti 39, 2000, Ploiesti, Romania, Tel.:+40444175292, e-mail: vulmanu@mail.upg-ploiesti.ro; fax: + 4044175847

Introduction

In rotary oil-well drilling, drillstem components, especially tool joints (the connecting partsof the drill pipe), and drill collars are subjected to severe abrasive wear when rotating in theborehole. In order to reduce wear, especially through deviated sections of the borehole, hard metalis applied to a section of drill pipe tool joint boxes and as hard material bands on the outside surfaceof the drill collars, as presented in Figure 1.

Fig. 1.

Due to the high costs involved by the hardfacing materials and welding technology, the hardmaterials are applied only in the most exposed to abrasive wear areas. The geometry of the deposit,the form and the dimensions of the tungsten carbides particles are not standardized. The drill pipeproducers usually accept the form and dimensions presented in Figure 1. The depth of the depositvaries from 1.6 to 3.2 mm. The single standardized prescription in the literature stipulates that anywelding technology would not affect the parent metal microstructure, [6].

Although hardfacing materials protect the drill pipe components from wear the casing isseverely worned, especially in deviated boreholes.

The hardfacing technique must then achieve the following two objectives:- extended life of the tool joint, and

- reduced casing wear.For minimum casing wear, the hardfacing should be flush with the tool joint outside

diameter instead of raised above the outside diameter. In the same time, the casing wear isinfluenced by the initial surface roughness, because there exist a relationship between the contactpressure and casing wear rate. Therefore, the tool joint should comply with a smoothnessspecification, which can be defined in terms of load-carrying area.

The above mentioned properties of the deposits can be achieved by appropriate selection ofthe hardfacing materials and technology.

For hardfacing of drill stem components, the welding processes are preferred because theyallow relatively thick coatings, with high strength bond between the hardfacing and workpiece.

The properties and quality of welded hardfacing deposits depend on welding process andtechnique, as well as on alloy selection.

An experimental program was dedicated to investigate hardfacing materials and technologyon a specially designed hardfacing equipment by using Plasma Arc Welding (PAW) and Gas MetalArc Welding (GMAW), both well suited in plant hardfacing of drill stem components. The paperpresents those experiments and conclusions drawn therefrom.

Hardfacing Materials

The hardfacing material is selected primarily by wear conditions met in the borehole (severeabrasive and adhesive) and cost considerations. The following environmental and manufacturingfactors must be also considered:

• impact, corrosion, oxidation and thermal requirements;• weldability;• deposition process.Owing to the friction in the contact area between tool joint and rock, the tool joint surface is

loaded by both cyclic normal and shear stresses. Fatigue of the bonding layer between the depositand base material may result in breakout of hardfacing material.

In order to fulfil the above mentioned requirements, the hardfacing materials are compositesconsisting of the following:

• tungsten – based carbides;• matrix.The matrix alloy is selected based on the following requirements:• toughness;• relatively high resistance to abrasive wear;• low sensibility to fragilisation due to dissolved C and W;• corrosion resistance in drilling fluids.As matrix materials are usually used iron-based alloys due to the compatibility with the base

material and low cost. The main disadvantage is the brittleness caused by dissolution of C, W andother elements following the decomposition of tungsten – based carbides.

The tungsten – based carbides (WC + W2C) are mainly used due to the high hardness andmelting point.

Welded hardfacing deposits are minicastings representing mixtures of tungsten carbideparticles embedded in an alloy steel matrix. Any dissolution of carbides results in a stronger butmore brittle matrix, depending on the process.

In the present research two types of tungsten carbide were used for carbides– steelcomposite weld:

• cast – crushed (with sharp corners);• sintered – crushed (with sharp corners).The size of the tungsten carbide particles ranges from coars (0.8 mm) to fine (0.35 – 0.4

mm).

The matrix material is a low carbon tubular rod steel with flux core, in spooled continuouscoil. The flux core conduce to reduce the carbide dissolution, increases the arc stability and reducesthe protecting gas flow.

Hardfacing process and facility

The hardfacing welding process is selected based on form and composition of hardfacingalloy, physical and metallurgical properties of the base metal and economic considerations.

The hardfacing technology was studied for two steels: AISI 4137H used for tool jointmanufacturing and 4145H used for drill collar manufacturing (low – alloy and medium carbonseels). Both materials are as quenched and tempered to 285 – 340 HB.

The chemical composition and the mechanical properties of the matrix material aredetermined by the chemical composition and the volume of the molten deposit, the temperature andthe duration of the hardfacing process as well as the amount and rate of the carbide dissolution. Anypartial dissolution of the carbides results in a stronger matrix. A matrix containing high amounts oftungsten and carbon may undergo martensitic transformation on cooling, which improves resistanceto abrasive wear but, in the same time, reduces toughness. The carbide dissolution is influenced bythe process as well as by the particle dimensions.

The wear resistance and toughness of the hardfacing alloy degrade as dissolution increases(the percentage of base metal in the hardfacing deposit). Then the hardfacing process must achievethe following requirements:

• perfect bond between hardfacing bands and the steel of the piece;• minimize deleterious martensitic reactions in the heat – affected zone by preheat and

postheat;• control the dissolution of the carbides due to high temperatures and oxidation;• control the microstructure and mechanical properties of hardfacing deposits by

solidification kinetics;• reduced costs.Two welding processes were studied for hardfacing of drill stem components with carbide –

steel composite: Inert Gas (Argon) Metal Arc Welding and Plasma Arc Welding.The pieces were hardfaced using a dedicated, specially designed, semiautomatic hardfacing

equipment, presented in Figure 2.

Fig. 2.

1. Turning mechanism2. Tool joint3. Gun holder4. Tungsten carbide delivery tube5. Gas metal arc welding equipment6. Carbide particles distributor7. Vertical positioning device8. Dc power supply9. Argon tank10. Flowmeter regulator11. Drive assembly control box12. Wire reel13. Flexible casing for welding wire and shielding gas14. Motor15. Longitudinal positioning device

The design of the equipment has to fulfil the following requirements:• realize an inert-gas shielded consumable electrode welding process;• control tungsten carbide delivery;• control rotation of the piece;• electrical preheating and stress relief of hardfaced pieces.

The preheat and stress relief parameters

The purpose of preheating is to decrease heat input and the rate of cooling in the region ofthe pearlite nose after welding. Because the IWS methodology for determining the preheatingtemperature is not applicable due to the high value of the carbon equivalent, in order to determinethe preheating temperature the Seferian method was applied:

CCt epr0' ,25.0350 −= (1)

Ce’ = Ce (1 + 0.005 s) (2)

where:Ce is the carbon equivalent;s is thickness of the piece.

The carbon equivalent is calculated with the equation:

(3)

For the above mentioned steels, the following results were obtained:

Ce = 0.558…0.766tpr min = 231…2910 C

It is also usual to select the preheating temperature superior to the temperature of thebeginning of the martensite transformation, determined by the equation:

451520SiVMoCrNiMnCCe ++++++=

Ms = 550 – 360 Ce (4)

obtaining: Ms = 275…3500 C.The selected preheating temperature for the hardfacing process was 325…3500 C.After hardfacing the stress-relief annealing is recommended in order to eliminate the

residual stress. The Hollomon-Jaffe parameter is:

H = (tD + 273)(20 + lg τD). 10 –3, (6)where:

tD is the stress-relief annealing temperature, in 0C;τD is the holding time, in hours.

For the recommended value of H = 16.5 – 17.5, which conducts to an important reduction ofresidual stress, and selecting tD = 5500 C, the holding time is 1.1 hours.

The operating conditions for hardfacing were firstly analytically estimated and thenexperimentally determined to the following values:

• tungsten carbide is fed onto the molten hardfacing pool imediately behind the torch,Figure 3;

• direct current, electrode positive;• consumable electrode wire diameter 1.6 mm• curent, A 280…320 (300…360 by PAW)• voltage, V 26…28• rate of wire feed, m/min 6…10• welding velocity, m/min 0.5…0.8• argon flow, l/min 15…18• tungsten carbide flow for 0.7…0.8 mm

particle dimension, representing 60…70 %of total volume of the deposit, g/min 300…340

• amplitude of the oscillation of the torch andcarbide delivery tube, mm 20

• bead width, mm 25• number of passes one for one bead;

each succesive bead overlappedthe preceding bead for 3 mm.

Fig. 3

The effect of welding process on the hardfacing characteristics

The quality of the hardfacing layer was evaluated by hardness and microstructure analysis.The repartition of the carbides at the outside surface of the tool – joint, Figure 4 and along

the depth of the deposit, Figure 5 is uniform.

Fig. 4 Fig. 5

Figure 6 (magnitude 40) reveals that each tungsten carbide retains the initial form withsharp corners, without cracks and voids.

Fig. 6 Fig. 7

The microstructure of the matrix consists of metal dendrites oriented rectangular to the basemetal, Figure 7.

The hardness of the carbide particles is 1400…1800 HV5 confirming that they were notaffected by the welding process. The hardness of the matrix is 550…680 HV5, confers a high wearresistance.

The analysis of the heat affected zone reveals the depth of aproximately 0.8 mm with ahardness of 550…600 HV5, which decreases to the value of the base metal.

In some samples cracks were observed in zones with smaller carbides, indicating thesehigher tendency for melting and subsidary alloying the matrix.

Conclusions

1. The Metal Inert Gas Arc Welding and Plasma Arc Welding processes conduct to thesame results for hardfacing deposits.

2. The hardness of the matrix and, in the same time, the susceptibility for cracking increasiswith the amount of dissolved carbides.

3. The susceptibility for matrix cracking increases with the amount of carbide particlesembedded in the matrix due to the probability of interrupting the matrix continuity. Theupper limit of carbide content is about 65…70 %.

4. Among optimised operating parameters of the hardfacing process the curent is one of themost important, due to the influence on the molten pool viscozity and possible carbidesegregation.

5. The tungsten carbide cast and crushed W2C and W2C/WC with dimensions 0.7…0.9 mmbehave better in comparison with sintered carbides (WC with Co). They maintain theform with sharp corners and initial hardness after the melting process.

Literature

1. Best B., Casing Wear Caused by Tool Joint Hardfacing. SPE Drilling Engineeering, Feb.1986, p. 62-70.

2. True M.E., Weiner P., Optimum Means of Protecting Casing and Drillpipe Tool JointsAgainst Wear. J. of Petroleum Technology, Feb. 1975, p. 246-252.

3. Schoenmakers I.M., Casing Wear During Drilling: Simulation, Prediction and Control.SPE Drilling Engineering, Dec. 1987, p. 375-381.

4. Marx C., Retelsdorf H.I. and Knauf P. Evaluation of New Tool Joint HardfacingMaterial for Extended Connection Life and Minimum Casing Wear. SPE/ADC 22003,Drilling Conf. Amsterdam, 11-14 March 1991.

5. Metals Handbook. Ninth Edition. Volume 6. Welding, Brazing and Soldering. ASMOhio.

6. Norme Internationale ISO 3962. Materiel et equipment pour les industries du petrole etdu gas naturel. Racords des tiges de forage en acier pour puits de petrole ou de gaznaturel, 1977.

7. Salagean T., Electrical arc welding (in romanian). Ed. Facla, 1977.