The Plasma-Master Co., Ltd. has developed the technology and also materials and the equipment for PTA-surfacing of extruder and heat softening device screws. The offered technology may be used with success both at manufacturing of new details, and at restoration of the worn-out parts.

Essence of a method
The essence of a method consists in deposition of wear proof and corrosion-resistant material directly on a crest of turn of screw.

The plasma transferred arc is used as a source of heating, the small-grained powder blown into an arc by a stream transported gas serves as an additive. Working gas - argon.

Materials
Due to application of powder that is specially developed for these purposes from the Fe-base alloy PM21, the surfacing is conducted without preheating of part practically in all range of diameters from 32 up to 320 mm. It essentially simplifies technology of surfacing.

Service life of screws surfaced by PM21 alloy raises in comparison with nitrated screws in 3-5 times. PM21 alloy replaces expensive Stellite successfully that is used abroad traditionally.

screw after use

screw after surfacing a - macrosection of surfaced screw turn without vibration of plasma torch b - macrosection of surfaced screw turn with vibration

Characteristics
Surfaced metal combines high wear resistance, durability and the plasticity, allowing to avoid occurrence of cracks not only during surfacing but also in exploitation. It is not inclined to tear and works with a nitrated layer of the cylinder well. Hardness of metal after surfacing is 43...45 HRC. After tempering at t ° = 650 °С, 1h its hardness raises up to 50...53 HRC.

The sizes of surfaced coat can be adjusted over a wide range, changing of quantity of supplied additive powder, amplitude of vibration of plasma torch and speed of surfacing. Due to good formation of surfaced metal the minimal allowance on the subsequent machining is reached. At correctly chosen surfacing mode the share of the basic metal does not exceed 5 %.
The rate of use of an additive powder is 95 %.

Equipment

PTA system based on PM-300

The equipment created on the basis of the PM-300 PTA surfacing module is used for PTA-surfacing of screws. The screw-cutting lathe or any other mechanism may be used for detail rotation and device moving relative to the turn. The type of the machine is usually caused by the maximal sizes of surfaced parts.

One operator carries out the surfacing mainly in a semi-automatic mode. However for surfacing of parts with complex geometry of a turn or in places of transition from one step on another, surfacing may be conducted in a manual mode. Connection of additional PP-25 manual PTA torch is provided for this purpose in the PM-300 PTA surfacing module.

PP-25 application

Many years experience of this technology using has shown, that it is a highly effective, reliable and economic way to increase the durability of a part.

Also see:
PM-300 PTA surfacing module
PP-25 - PTA torch for manual surfacing
PMalloy 21 - Wear-resistant Fe-based alloy
services: PTA-surfacing
science: Plasma transferred arc surfacing (PTA-surfacing)