Drill collars are made with material AISI 4145H in accordance with API Spec 7-1. We supply slick drill collars,  spiral drill collars and short / pony drill collar; their sizes are 30 feet  and 31 feet.
 

Steel grade:
AISI 4145 H modified alloy steel, full length heat treated, with chemical analysis and mechanical properties which conform to the requirements of API Spec 7 (latest edition)

Types:
- finished drill collars
- slick or spiral
- with or without slip + elevator recess
- bored drill collar bars, plain end

Drill collars are made of self-designed premium alloy steel bar and pipe materials, and heat treated by continuous gas furnace to meet the mechanical requirements in API Specs.  

Drill Collars

Slick collars

Spriral collars

Flex collars  

2, Elevator, Slip Recess or Slip Holes

Unless otherwise specified, those are machined according to API RP7G recommendation.

3, Stress Relief Groove

4, Cold Rolling of Thread Roots

5, Make In & Break Out

6, Hardbanding 
7, Thread Surface Process

All threads will be treated with phosphate coating process to improve wearness and prevent galling.